Well it depends, they try to update us on PlayStation Blog either weekly or monthly about what is coming and which games are leaving. On the PS Store there is a section for last chance that will inform you of items that will leave soon. As with any digital item you can "purchase" the item and decide to download it later as it will remain in your download list. So even if you run out of space you are fine to pick up the tons of free content Plus offers.
Games like InFamous 2 and LittleBigPlanet 2 have been on the instant game collection since it started, but we've seen games come and go. Something like 48 a year or something is their target, with a healthy mix of full retail games and PSN only titles. Ideally there will always be 12 games (for SCEA) that are available as the Instant Game Collection, but sometimes it might be 11 for a week while they play catch up with the new free games. What content they are bringing is subject to change and delays, plus they don't always know what's coming that far in advance.
As long as you have plus, you will have access to any free games you get off the store. Just snag them up as soon as they become available on the weekly update so you don't miss out on a title that might only be available for a week or two or three, as a few have done that.
Before the PS3 everyone was nice to me :(
