Spongebob usually hangs out by the "Totally Nickeloden" store (next to Shrek 4D and the Nick Blast Zone)..he even has a special back drop you can pose with even when he's not there....
Sponge is usually out most of the day, with the occasionaly 5 minute break here and there (same with Shrek)...but if you don't see him, just ask an employee in the area...they should know when he's coming back.