90% seems to the rate everyone comes up with when they actually measure it, judging from various independent games blogs and feedback from acquaintances in mobile gaming. I wouldn't be surprised if the rate holds for AAA titles as well.

On the flip side, I recall at least one set of statistics showing that most pirates don't run a game more than once. Presumably most pirates pirate huge numbers of games and simply don't have the time to play them all in depth -- which certainly supports the idea that they wouldn't pay for them if piracy weren't an option.