http://www.j-right.com/html/ccfl_pc_cooler.htm
There's one example of a fan's rated approximate lifetime. Some will run more, some will run less. If you can open it up to lube it, then it will last even longer. But think of it this way: 50,000hrs/24hrs = 2083.33 days. That means 2083.33days/365 days = 5 years, 8 months of 24/7 use. Chances are your PC will be replaced in less time.