Sacrifice the inhabitants of the Material Plane
Death Knell (and similar) is the premier method to arbitrarily boost CL at the cost of many, many lives. Each sacrificed creature provides you with +1 CL for 10 minutes per HD. Assuming you're level 20 and want a spherical realm 1000 miles in diameter (it's a small moon, but it'll do), you 'only' need approximately 400 quadrillion sacrifices (or 400 billiard, if you're British). That sounds like a lot! Let's see how long it would take us to set up this casting. (in the end, we're gonna end up only needed a 60th those guys because we're going to prestige class into eldrich knight and do the actual casting in a single round instead of 6 hours, but that just decreases our target by 1.5 orders of magnitude, which doesn't affect the calculations much-- things will take maybe 6 rounds less time)
N.B. Widen spell doesn't help us here, cause the effect isn't the right sort :( If it was, we could spare over one hundred quadrillion people's lives. Oh well!
N.B. I'm told Death Knell's cumulative bonus is somewhat controversial. If the spell is banned in your game, or functions too differently, this answer won't work.
So, the most efficient way to set up a delayed sacrifice in Pathfinder is definitely Glyph of Warding. Death Knell is a harmful spell, you throw it in the Glyph, anytime later any of your clergy can go grab some sacrifice and throw 'em on the altar and boom, you get +1 CL. You don't need to be on the same plane, you don't need to be doing anything special, it just works. Yay!
So, you get yourself at-will Glyph of Warding somehow (cause otherwise it's stupid expensive) and you're set.
The best way to do that is to get yourself an intelligent item compatriot. For the relatively low, low price of 132,000 gp, you can add Greater Glyph of Warding to a sentient item of your choice (Unfortunately, Glyph of Warding is one of those troublesome 3rd level spells that aren't allowed to be used at-will, so you can only do that via at-will Limited Wish, which is another 60K).
Now, that's just 1 Glyph of Warding a round, which would take millennia (70 million millennia to be precise) to finish. You can speed that up with a peasant railgun, but still, it's hardly practical. So, instead, we'll make an item that makes these items.
Enter Impart Mind. It gives you an intelligent magic item that lasts 1 hour per caster level. With just a few hundred thousand sacrifices (and Extend Spell), that's easily a century, which should be more than enough for our purposes (we hope!). Since this also gives us a modest bonus on our d% roll for powers, we won't be spending any extra gems or diamond dust. However, we do really need to be picking our Special Purpose (so as to avoid any unfortunate conflicts of interest), so this will have to be a Maximised Extended Impart Mind (normally a 9th level spell).
Now, to put this spell into a trusty sentient item buddy sidekick, it needs to be 7th level or lower. Impart Mind is normally at least a 5th level spell, so we're gonna have to cut that down a bit.
Metamagic Adjustment Time! We can use Magical Lineage to lower the level by one, which makes this work for a Magus (and magus only), as a 7th level spell. The rest of the metamagic adjustment stuff lets you cast it for free, but not write it into a spellbook that way. Wayang Spellhunter works this way, too, but only on low-level spells. Sad :( In any case, Extended Maximized Impart Mind is a 7th level Magus spell (which isn't actually castable) for a Magus with Magical Lineage. By taking two levels of Pathfinder Savant as a multiclass Wizard, we can take that Magus spell and put it on the Wizard spell list at the same level, which can get us access to the 7th level version without having to be a Magus, which is good.
So, we get that thing in our spellbook as a known 7th level spell, we craft a single stupidly-high-CL intelligent item with the special purpose of serving us and our interests with that at will as the Dedicated Power (and probably other cool abilities too cause why not), and it starts popping out duplicates of itself that only last a century. Those duplicates start making duplicates, and things rapidly get pretty crazy. Assuming access to vast troves of cheap magic junk (such as Dull Gray Ioun Stones), we can keep the reproduction going without much issue, but the value of those Ioun Stones does quickly become astronomic. Instead, by giving the items Continual Flame as a lower level ability, we can ensure a steady supply of Everburning Torches suitable for awakening.
Interestingly, having the items cease reproduction to create the Glyphs of Warding creating versions isn't a good idea till we get almost to the end (they should shift production about 8 rounds before they finish). It takes us about 12 minutes from the first casting to produce the needed number of Glyphs this way.
Next, we need the sacrifices, and we need them dying. We can gate a number of 1 HD dwarven barbarians equal to twice our caster level with each Gate we cast. Sacrificing them, then, results in us possessing triple our previous caster level for 10 minutes. Blast Glyphs, unfortunately, deal a minimum of 5d8 damage, which is more than enough to kill a level 1 dude most of the time. This means we need even more metamagic.
The Glyphs of Warding previously mentioned need to actually be Merciful Glyphs of Warding, which doesn't change their spell level or anything. Just another feat we need to have, so as to 'know' the spell in that form for Impart Mind's purposes.
A Merciful Greater Glyph of Warding set to deal 5d8 non-lethal damage will kill the average 1HD NPC dwarven barbarian outright about 12% of the time, and leave them not dying about 9% of the time (and not unconscious <0.02% of the time, which is kinda a big deal, but can be dealt with via defensive magics). This reduces the efficiency of our sacrificial system by about 20%, which means we only multiply our caster level by 12/5 per casting of Gate.
Using Pearls of Power IX to replenish our 9th level spell slots (scrolls are cheaper and faster, but you need special stuff to get them to scale with your caster level), we can cast Gate lots of times. Fortunately, we only need to do so around 43 times. Unfortunately, we may need the sacrifices to keep dying for the whole 6 hours we cast Create Greater Demiplane, depending on GM. In that case we can either build an elaborate contraption wherein we, upon reaching sufficient CL, gate our victims in many many rounds of fall-time above the ground below, with conditions such that the fall does not, in fact, harm them, or we can just prestige class as an Eldritch Knight. An Eldritch Knight gains the Spell Critical ability at tenth level, which lets us cast Greater Create Demiplane when critical-hitting a target as a swift action as long as we attempt to drag them to the plane with us afterwards. Critting a target can be trivially done via a coup de grace once per round, so if you want you can get a bunch of big demiplanes in your 10 minutes of fame, but they would all need to be protected and permanented and stuff seperately, which is kinda what the whole point of doing this was to avoid.
And we're done! The process took around 165 rounds in character, which is about 17 minutes, plus whatever time gathering and slaughtering the initial Metropolis takes (prolly a couple weeks? idk. A campaign arc) or like a year if you just craft the first item from scratch.
So, the full build here is:
Wizard 3/Pathfinder Savant 6/Anything that grants Martial Weapon Proficiency 1/Eldritch Knight 10
or, if you can play a tiefling with variant racial abilities granting a 1/day 3rd level spell like ability (or are playing an Aismar):
Wizard 1/Eldritch Knight 10/Pathfinder Savant 6/Anything that Advances Wizard casting 2/Anything 1
With feats spent on:
- Extend Spell
- Maximise Spell
- Merciful Spell
And a trait spent on:
And Death Knell (which becomes spell level 2), Greater Glyph of Warding (which becomes spell level 7), and Impart Mind (which becomes spell level 5) as our esoteric spells.
It's not resource conservative, and it's not OoC time conservative, but it's really really time-conservative, and it scales better than anything else!