It's taken us anywhere from 6 to 12 weeks depending on the items addressed in the Service Pack and how many level we're moving. The MOST trouble we've encountered are in undocumented changes to fields names and functionality.
It takes usually 6-8 weeks to implement a service pack for us. We implement only 2 (max 3) SPs per year, so mostly it there will be a big jump in the versions. When a Service Pack is released, we evaluate all the fixes in it, and if it is fixing atleast 5 of our top priority issues, then we go for it.