Wizards must choose to apply the metamagic feat to their spells while preparing the spell? If yes, can they just cast spells with the metamagic feat this way?
Sorcerers and Bards
When sorcerers and bards cast a spell using a metamagic feat, they use a full-round action. That is ok.
Will they use then the previous and the new rank of the spell from their daily maximum? If they use more than one feat to a spell it take a full-round for every feat applied?
Clerics and Druids
What about clerics and druids using metamagic feats in their spells, they must prepare the modified spell before or can choose while casting? When they cast a modified spell, they take a full-round action?
In case Zachiel hasn't made it clear enough...
These Are Mainly Examples
Excludes corner cases.
The cleric, druid, wizard, and other casters who prepare spells must prepare spells that're to be modified by one or more metamagic feats, designating each spell to be affected by the metamagic feat when the spell is prepared. The spell occupies a single spell slot with a level equal to the spell plus additional levels equal to all the metamagic modifiers. Casting time is unaffected unless a byproduct of the applied metamagic feat.
- Example 1: The 1st-level Clr spell divine favor [evoc] (PH 224) modified by the metamagic feat Extend Spell (PH 94) occupies a 2nd-level spell slot. A cleric must be at least level 3 to prepare such a spell.
- Example 2: The 1st-level Wiz spell color spray [illus] (PH 210) modified by the metamagic feats Silent Spell (PH 100) and Still Spell (PH 101) occupies a 3rd-level spell slot. A wizard must be at least level 5 to cast such a spell.
- Example 3: The 4th-level Drd spell blight [necro] (PH 206) modified by metamagic feats Empower Spell (PH 93) and Maximize Spell (PH 97-8) occupies a 9th-level spell slot. A druid must be at least level 17 to cast such a spell.
The sorcerer, bard, and others who cast spells without preparation (including a cleric or druid using the class feature spontaneous casting) can choose to apply one or more metamagic feats they know to spell they are about to cast; this either increases the casting time for spells with a standard action casting time to a full round or adds 1 full round to the casting time of spells with longer casting times. Further, the metamagic modification changes the level of the spell slot used when the spell is cast to a spell slot of the spell's level plus that of the metamagic feat's modifiers.
- Example 1: A bard has a spell known the 5th-level Brd spell song of discord [ench] (PH 281) as a spell known and possesses the feat Still Spell (PH 101). He can either cast the spell song of discord as a standard action as a 5th-level spell or cast the spell song of discord modified by the feat Still Spell as a full-round action as a 6th-level spell.
- Example 2: A sorcerer has as a spell known the 7th-level Sor spell summon monster VII [conj] (PH 287) and possesses the feat Empower Spell (PH 93). He can either cast the spell summon monster VII as a 1-round action as a 7th-level spell or cast the spell summon monster VII modified by the feat Empower Spell as a full-round action followed by a 1-round action as a 9th-level spell.
- Example 3: A cleric who has metamagic feat Maximize Spell (PH 97-8) hasn't prepared the 1st-level Clr spell cure light wounds [conj] (PH 215-6). The cleric wants to use his spontaneous casting class feature to cast the spell cure light wounds and modify the spell with the metamagic feat Maximize Spell (PH 97-8). Because cure light wounds normally takes 1 standard action to cast, it instead take a full-round action to cast. The spell cure light wounds is normally a 1st-level spell but the feat Maximize spell makes the spell use a 4th-level spell slot instead.
It's almost all in the SRD, albeit it's in the feats section, where metamagic feats are detailen, and not in the spellcasting section where one could expect to find it.
This is the relevant part:
Wizards and Divine Spellcasters
Wizards and divine spellcasters must prepare their spells in advance. During preparation, the character chooses which spells to prepare with metamagic feats (and thus which ones take up higher-level spell slots than normal).
Sorcerers and Bards
Sorcerers and bards choose spells as they cast them. They can choose when they cast their spells whether to apply their metamagic feats to improve them. As with other spellcasters, the improved spell uses up a higher-level spell slot.
Spontaneous Casting and Metamagic Feats
A cleric spontaneously casting a cure or inflict spell can cast a metamagic version of it instead. Extra time is also required in this case. Casting a 1-action metamagic spell spontaneously is a full-round action, and a spell with a longer casting time takes an extra full-round action to cast.
The reason for sorcerers and bards needing more time to cast the spell is a balance one. Casters who prepare spells must choose in advance which spell they want to apply the metamagic feats on, which makes them occupy a higher slot.
A sorcerer or a bard can decide on the spot to never cast the quickened mage missile, casting a sixth level spell instead.
This flexibility is paid with the extra casting time. No rule tells this applied once per metamagic feat applied, so I think it's quite sure you don't need to.
As a consequence, Clerics and Druids (who prepare their spells) don't usually need the extra round, except if they apply the metamagic feats to the spells they cast spontaneously, such as the converted healing or summon nature's ally spells