Life expectancy is hard to estimate and many manufacturers quote different lifespans.. LiteOn for example quote 70,000 MTBF. Go on any forums and you can see people's brand new drives dying in next to no time while others have drives that are 3-4 years old and still going...
When I bought my Sony 510 I read that the tray for example had a life expectency of 10,000 ejections and the drives lifespan assumed a certain amount of the drives life was spent burning and reading hence 50,000 hours life doesnt mean 50,000 hours worth of burning.
From my experience I end up replacing a drive long before its end of life is reached (currently on drive number 6 and all the previous ones still work).
Of course heat is a big enemy of computers and you can keep your burner burning cool with a quiet fan inside your case blowing air at the bottom of the drive where the heat transfer areas on the drive case are (going from passive to active type cooling) which is useful if your popping out dvd's or cd's like mcdonalds churns out greasy burgers.
Modern burners don't heat up as much as the ones from last century and unless you live in a tropical area or you've got a sweatshop operation going on you should be fine.