Well I have a hard drive from a computer that was bought around 1990 Its a 2 GB drive I think, and it still runs strong. I believe that it all depends on how you treat the drive. I.e. Cold, hot, etc. I wouldn't be suprised to see a hard drive last 20 years.

As far as telling when its going bad? Well most of the time, you can't tell. One day it works, and the next day its dead. trust me I know. lol The only way to prevent complete data loss is to back up. Now if you start to hear clicking coming from the computer then you probably need to start looking for a new HD. lol