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.
Best Answer
The rule for Quicken Spell implies that swift-action spells do not have their casting time affected by metamagic. In the case of a spell that is swift because of Quicken Spell, other metamagic feats explicitly have no effect: the rules state that spells modified are entirely exempt from the casting time rules. That would include any casting time increases from other metamagic feats.
By comparison, then, to spells that have a swift-action casting time due to Quicken Spell, we can see that spells that have a swift-action casting time naturally should also have their casting time unaffected by metamagic. However, this is not clearly spelled out in the rules, and furthermore Paizo has explicitly declined to address the issue (see this FAQ candidate thread, marked “no reply required”), so this implication is the best we have available to answer the question.