It's impossible to answer this, really. I'll assume you are talking about a manual transmission. But, after that, we'd need to know the kind of vehicle, the kind of loads it carries, what is the normal driving terrain like (hilly?) and the habits of the driver.
Here are a few things that can shorten the life of a clutch:
- Driving with your foot on the clutch, even if you are not pressing it. Resting your foot on the clutch takes out the free play, which means your clutch is probably slipping, though you may not notice it
- Not keeping the clutch adjusted properly so it has free play in the pedal
- Carrying a lot of weight constantly will put strain on the clutch
- Driving in hilly conditions or having to make stops on inclines (e.g. San Francisco driving) a lot.
Under average conditions I'd say a manual clutch should last 5 years plus or minus.