With the exception of taking more levels in Paladin, is it possible to increase my paladin special mount duration.
The only direct effect of this sort I can find is Sacred Steed from Dungeon vol. 104. It requires you worship Azor’alq, and gives a +1 bonus to your effective paladin level for the purposes of your special mount (i.e. another 2 hours), and more importantly, it allows you to call the mount a second time per day, effectively doubling the duration.
The call mount 2nd-level paladin spell from Book of Exalted Deeds can alow allow you to summon the mount again after you’ve already used your 1/day summoning, effectively resetting the duration (and as early as 6th level, two summonings per day covers the entire day). Unfortunately, call mount is a 1-round casting time spell, so it’s best used outside combat if at all possible. Even the excellent Battle Blessing feat won’t help there. Note that a wand of call mount is a relatively affordable 6,000 gp (if you went with an arcane hierophant build, we could even make this an eternal wand), and can be used even before you have 2nd-level spells or even spellcasting at all.
Beyond those, well, you can increase the mount’s duration without taking paladin levels, but that probably isn’t really what you meant.
A large number of prestige classes all progress a special mount.1 In addition to those, the Holy Mount and Theurgic Mount feats from Dragon vol. 325 allows any divine or arcane spellcasting class, respectively, to stack with paladin for the purposes of the special mount, and the High One warrior wizard substitution levels from the Champions of Valor web enhancement also allows the wizard class to do so (and combine their special mount and familiar, which is nice).
The Harmonium peacekeeper (Dragon vol. 315) and the ranger-knight of Furyondy (Dragon vol. 317) both deserve special mention, though, because they add a +4 and +5 bonus, respecitvely, to your effective level on top of stacking with paladin levels. So with these:
5th-level paladin/1st-level ranger-knight: special mount of an 11th-level paladin, 22-hour duration, at character level 6th.
6th-level paladin/1st-level peacekeeper: special mount of an 11th-level paladin, 22-hour duration, at character level 7th.
5th-level paladin/1st-level ranger-knight/1st-level peacekeeper: special mount of a 16th-level paladin, 32-hour duration, at character level 7th.2
If you’re serious about that mount, Harmonium peacekeeper is a level you probably want, since the requirements are dead-easy, too (most 6th-level paladins with any interest in their mount will qualify even if they weren’t specifically planning on the class). The ranger-knight of Furyondy is much, much more painful to enter unless you take a couple levels in ranger for Track and Two-Weapon Fighting.3
Ultimately, though, I have to wonder why you need this. The duration on the paladin special mount is massive; even when you first get it at 5th, it lasts 10 hours. Not many adventurers are active for 10 hours a day (the rules of the game naturally encourage a day divided in thirds, 8 hours each spent active, downtime, and sleeping), which the mount more than covers. The various prestige classes and other things here actively improve the mount you have in significant ways, which can be useful, but it seems like they never bothered just extending the duration because really, no one really needed that. The duration is quite long as it is.
Namely, ashworm dragoon, Aglarondan griffin rider, bone knight,1 cavalier, halfling outrider, Harmonium peacekeeper, knight of the Blue Moon, knight of the Iron Glacier, ranger-knight of Furyondy, triadic knight, Vadalis beastkeeper, windrider, and wild plains outrider.
Bone knight actually replaces your special mount with a skeletal steed, but the skeletal steed works exactly like a paladin special mount and gains special mount benefits based on your paladin level + bone knight level, so it still gives increased usage of a special mount.
I have actually played this kind of character, and I decided to self-nerf because having an effective level so high seemed unfair. I was extremely disappointed in the character; if you really want your special mount to be a focus, it needs to have these super-high levels to be serious combatant. Less necessary if you’re just looking for a reliable mount just for your own mounted combat abilities.
If you do take levels in ranger, the Devoted Tracker feat from Complete Adventurer allows you to name your special mount as your animal companion, so it gets both sets of bonuses, partially replacing the couple of paladin levels you lose. Natural Bond, also from Complete Adventurer, gives a +3 bonus to your effective level for animal companions, to boost those bonuses a little more. Halfling outrider both advance animal companions and special mounts at the same time, and there’s a halfling druid substitution level that improves your animal companion’s ability to be a mount (a level in druid to get it would slow down entry into ranger-knight of Furyondy and/or Harmonium peacekeeper, though, since they require BAB).
A sample build that takes advantage of these options to have an exceptionally powerful mount.
|1st||Paladin||1st||—||—||Mounted Combat, Trample¹|
|3rd||Moon-warded Mystic Ranger||2nd (3rd)²||0th||—||Devoted Tracker|
|4th||2nd (4th)²||1st||—||Armor of the senses|
|8th||Fangshields Ranger 4th||17th||4th||+10|
|10th||Focused Animal Druid||19th||11th||+14|
Pal. and Dru. refer to effective paladin levels for special mount and effective druid levels for animal companion, respectively.
HD refers to the number of bonus hit dice we get for our animal companion-mount, assuming we use one of the basic options for special mount.
Indicates a bonus feat. These mostly come from the class level that the feat is listed with, but at 1st level the feat is from race or flaw.
The parenthetical value indicates what that level would be worth once Holy Mount is applied, even though Holy Mount is not available at that point.
The values of effective paladin or druid levels above 20th assume the use of the Epic Level Handbook rules. Since these advance the special mount/animal companion very slowly, it doesn’t really matter a whole lot (4 HD).
In addition to the ideas above for mounts, this build utilizes Devoted Tracker and animal companion options: beastmaster, fangshields ranger, and focused animal druid all offer bonuses similar to animal companion similar to those peacekeeper and ranger-knight offer special mounts. Focused animal druid, in particular, is awful in general, but for this purpose it’s solid enough.
The other variants are mostly just gravy: mystic rangers have far better spellcasting, and moon-warded rangers get that sweet Wis-to-AC even in light armor. There are fangshields druid substitution levels as well, which are decent and some of them might be compatible with focused animal. The Unearthed Arcana druid variant is interesting with moon-warded ranger (2×Wis to AC), but incompatible with focused animal.
Fangshields ranger requires being non-humanoid, so choose race accordingly, or become a necropolitan because why not? We qualify for ranger-knight of Furyondy with a pair of gloves of the balanced hand from Magic Item Compendium, which grants Two-Weapon Fighting. A battle bridle, also from Magic Item Compendium, could get us Mounted Combat as well, but at 9k it might be mildly awkward to justify owning one in time to take Trample, so I favor just using flaws for those feats.
Yes, but you'll need a friend.
Psionics has an answer for you, the 5th level Nomad power Temporal Reiteration. It causes the "past round" not to count against the duration of all onging effects. Augment it to share with a friend (assuming that the paladin doesn't also want to be a mid-level Psion or Erudite.
Famously combined with the Font of Power class feature of the Metamind which grants free power points for one minute. So long as nothing prevents you from manifesting the Temporal Reiteration each round, (and there are a number of things which can...) you can stretch that minute out and play Energizer Bunny till you drop. Frequently combined with the Linked Power feat to enable the manifesting of non-quickened or swift/immediate action powers.