It depends on the game what makes sense to give for free vs. hold back for purchase, but the general rule of thumb is the free version is around a quarter to a third of the full version levels, and when the player completes the last level you advertise the full version.
Exactly which levels to give has a couple schools of thought. On the one hand, most people advocate simply using the first N levels as your free version, so that the game’s progression (both the story and how the levels ramp up in difficulty) is presented in the correct order. Other people however advocate using levels spread out throughout the game, so that the player gets to experience what later levels are like. This only really works if later levels don’t depend on the player having gone through earlier ones (you definitely don’t want the levels to be too difficult and they give up) but I see this done a lot in games where you get new powers or something later (eg. Angry Birds shows off all the different birds.)
Not every game has levels though, so other other common limitations are a time limit or something like that. Just be careful that Apple can be kinda persnickety about limited versions of apps; their guidelines say no greyed out buttons so people won’t be disappointed by expecting something to happen. [import]uid: 12108 topic_id: 5384 reply_id: 17982[/import]