A creature can generally notice the presence of an active invisible creature within 30 feet with a DC 20 Perception check. The observer gains a hunch that "something's there" but can't see it or target it accurately with an attack. It's practically impossible (+20 DC) to pinpoint an invisible creature's location with a Perception check
So at start of fight it is a free action but on next rounds to pinpoint target it is a free or move?
Action
Most Perception checks are reactive, made in response to observable stimulus. Intentionally searching for stimulus is a move action.
Move action. Only the first attempt is free.
edit: If the invisible creature does something that counts as observable stimuli to me so you get another free attempt. But if he does not do anything then no more free attempts. Basically you get one free perception check per action the opponent takes.
As an example if it moves and/or tries to hide that would get you a free attempt.