how long should it take to replace a timing belt and a water pump at a service garage?

This is routine maintenance, but it is not an easy job if done correctly- I don't know the quoted times for your car, but I'm guessing it shouldn't take more than 4 hours- unless that car is diabolically hard to work on- ask the service adviser- they should be happy to explain it to you- if they don't want to talk, take it somewhere else-

A shop will quote you 8 hours, even if it only takes them 4. You will pay for 8. That's how they make their money. If they are going to stick you for 8, make sure they replace all the parts while in there. Not just the pump and belt. Make sure they put in new idler pulley and tension pulley also. That's how they also make money...skimp on the parts........

Id be real surprised if it really took even a hour so charging for 2 hours is plenty. My shop charges 85 pr hour so should be no more then 170 bucks labor. Best buy is partsgeek.com