Some people claim it's around 15. I never count, but some cards can double or triple that with ease.
My guess is 15 to 30 based on the end card counts I see. And yes, easily affected by the available cards. Someone should rate the cards based on how long certain ones can extend the length of the game and secondly, the length of turns. Usually a card that extends the length more, also increases the length of the game, but I could see there being exceptions to that.
In any case, I know that multiple buys affect the card count, but that varies too.