The game is 50 levels long. Each level takes about 6-10 minutes You will fail a few levels on the first try, maybe 10 out of the 50. So the playtime is anywhere between 6-10 hours. But that's without replaying through the game.
The only minigames in the iPod version are the ones you encounter in between missions, there isn't a separate minigame-only mode
there is after you finish the game- it's called quick play you can play the minigames separately, as well as the levels with all your newly discovered plants