I will pitch hit for the cosplay coordinator here until she gets onto the forums... Cosplay skits are going to be limited to three minutes max, but how strictly that is enforced depends on the number of entrants. Truth is, that time is really meant so that groups can do decent skits. If you are up there alone and doing a "walk-on" to simply enter your costume, yeah, you should probably be thinking 45 seconds to a minute of stage time at most, unless you have some kind of routine to do or monologue to give to spice it up for the audience. Cosplay's a show as much as a contest.