It's usually the 3rd party full game retail titles that move out pretty regularly (monthly). The 2nd party Sony stuff has pretty much been there from the day the Instant Game Collection started. The pattern seems to be a full retail PS3 game the first couple Tuesdays of the month and then PSN games the following 2.
Bioshock 2 should be leaving this Tuesday and then SSFIV the following week. No idea what will be replacing those until the day before; that's usually when when the Official PS+ Blog gets updated.