How long does the cooling fan inside a MBP last?

I have a 2009 model. Since Fall of that year I've been using it mostly for computer animation and since Spring of last year, I've been using it to play games like Team Fortress 2 on Steam and Deus Ex: Human Revolution on OnLive. The cooling fan inside my MacBook Pro spins up to around 5000 rpm when I play my Steam games to keep it below 100 degrees celsius.

I feel confident that it may be ok to play games as I meet the requirements but how long would the fan inside my MacBook Pro last? I never bought Apple Care unfortunately, so how much would it cost to have it fix just in case the worst happens?

Replacing the fan and sensor module in a MBP is a do it yourself job if you have any kind of "repair skills". You can get a pretty good idea of the procedure and cost by going to iFixit: The free repair manual with your model number.

As to how long the fan will last depends on how often it's maxed out or operates at high RPMs. Folks who are ardent gamers with their Macs (especially the portable models) tend to wear the fans (and the machine itself) out faster than those folks who just use their machines for everyday things.