I usually play a fifth of the pots (if not a tenth) if it's a big table (7-10 maybe), because people tend to be aggressive, bet high, and someone will probably have a way better hand than me.
In 10 person tournaments, I don't even start playing until about hand 5, because someone always goes all in by then, and I don't want to call them if I don't have fantastic cards.
I probably play to some point in the hand about 25%-33% of hands in a small game.