For the purpose of discussion, the spell I have in mind is Chill Touch.
When you spend your charges, is that a specific kind of action, or are you just spending one charge per attack under whatever conditions apply (full attack, haste, AoOs, etc...)?
Spending a charge per se isn't an action, it happens whenever you touch someone, e.g. because hit him with an unarmed or natural attack.However, you now have the option to do touch attacks. Doing a touch attack is a standard action, so you only get one per round.