A lot depends on how the car is used. If a car were only driven on the highway, a manual trans could probably almost last indefinately.
If you do a lot of shifting, it seems the first thing to go is the first gear synchro, and the second gear synchro. These are the parts that have to work the hardest on a trans.
Anyways, I got over 200,000 on my Porsche 914, and it still worked pretty well when I had it rebuilt.