If you need to formalize it, Normal Change should be at least a move action to parallel similar effects. Instant Up allows you to get to stand from prone as a Free Action instead of a Move Action, so Quick Change is just that logic applied to changing costumes.
Another existing mechanic would be Quickness Limited to One Task: Changing Clothes. This costs 1p for 3 ranks of Quickness, and since Quickness defines Free Action as rank -2 time or less, working backwards makes Changing take up to rank 1 time, or 12 seconds, which is 2 rounds as was suggested above. You could even define this further as 4 Move actions, allowing for some comedy where you can intersperse attacks, movement and stealth with getting your costume on.
So informally but derived from published definitions, changing clothes should take 3-12 seconds, or 1-4 Move Actions. Done under pressure, this essentially turns getting dressed into a Challenge that might call for some Dex checks.