If the spell’s normal casting time is a standard action, casting a metamagic version is a full-round action for a sorcerer or bard. (This isn’t the same as a 1-round casting time.) The only exception is for spells modified by the Quicken Spell metamagic feat, which can be cast as normal using the feat.
For a spell with a longer casting time, it takes an extra full-round action to cast the spell.
The rules only cover spells which have a casting time of a standard action or greater.