Changing the pump can take up to 4 hours (average mechanic)! that all depend
how much you/he will have to deal with rusted bolt.

I did 5 x 2.1L water pump replacment, the best i can do is about 3 hours
including the removal of the coolant. But that is if everything is perfect.
Average time would be 3 to 4 hours for removal, another 2 for bleeding.

Removing the coolant and bleeding the van is the longest parts.

Cheers, Ben
http://www3.sympatico.ca/huotx/water_pump.htm
http://pages.infinit.net/eurokit/