I'd say a manual should outlast the car. Synchro rings might require changing but in normal use the box itself should not break.
However, it seems that lately many manual boxes are failing with minimal mileage, I think one reason for this is this new fad of trying to achieve every driving move in 5th gear.
Unless one can round a city corner in 4th at least some magazine is going to bitch about it. This kind of usege leads to insufficient oil flow and eventually to failure. Witness, say Ducatos or Volvos, both seem to munch their gearboxes quite regularly and at least in Ducato's case using 5th below 80 km/h seems to be the recipe for disaster.
Obviously cost cutting and low pulling diesels are a factor as well.
Cost cutting seems to have the same effect on European autoboxes as well, whilst a good old yankee box will go on forever with occasional oil change, a premium eurobox will most likely detonate well before the car hits any serious mileage. Sealed for life my aXXe.
-Tazio