Yea it really just depends.
We can sit here and say X amount of time, but in reality the way products, specifically electronics are manufactured its always a roll of the dice.
You can however, if you have the drive already, is get a S.M.A.R.T reader. I know Speedfan has one, and what it does is tell you the integrity of your hard drive. And even gives you an analysis of your drive and how it should perform.
A lot of new hard drives have this technology. So if you have a drive and feel something is wrong that will let you know if there is.