I bought my car at 91000kms , did a cambelt change then and other normal tuneups but the waterpump was left out.Mechanic told me at the time that it didnt need to be changed since it was fine and I went along since I didn't know much about cars.

Is it just a matter of time before the waterpump would fail or are they supposed to last. What are the implications of it failing? Overheating?

From what I gather, if I was to get someone to change just the waterpump now, it would cost quite a bit?How many hours of labour would it be.

water pumps are tough and don't wear out often. Its a rule of thumb to change the water pump and timing belt together to save on the cost of labor.

The cost to changing a water pump would be around the same labor cost of changing the timing belt plus the cost of the water pump itself.

When a water pump fails it would rattle and make alot of nasty noise before it would fail and over heat your engine.

There is no say how long a water pump will last, I've seen a car have the factory water pump for 150,000 miles before the owner decided to do a tune-up.

I just did a service at 99500kms and didn't change the waterpump. The mechanic told me that it was rocksolid! I said that when he was about to change the timing belt that he also should change the waterpump, but he said it would last a long time. (I hope)

I would have just changed out the water pump for safety reasons. For the 5S engine, I do know that if the water pump stop working, it would jam up, not spin, and actually eat up the timing belt. So, If you just changed out your timing belt with a new one, expect to get another one from Toyota. Thats what happened to my camry. The water pump just stopped spinning and the belt just slashed it self trying to go across the cam and crankshaft. At the end I just bought a Life time water pump and a new belt. Sucks not to do it again, but to buy the same things again and something extra.