How long will it take to replace the water pump

Took me four hours on my '88 but that one has WAY better access than on a '99 model. Expect it to take around six hours. If your mechanic is conscientious and looking out for you in the long run, he will recommend replacing the timing belt at the same time. That will require a couple extra hours but over half the work will already be done.

The 3.0L is not an interference engine so no valve damage will occur if the timing belt breaks, but it's silly to let that maintenance item go when the job is three quarters done already by doing the water pump.