How long should it take to build a gaming desktop at a local computer store?

I would say a few days. If the store is good, they will normally do at least a 48 hour burn test. No good store will send you on your way without a good burn test. When I build gaming rigs for my buddies, I usually run through a few tests and benchmarks after the burn to make sure all the temps are low and all the parts are behaving (eg. no extra noise). All that takes time. Never accept a rush job if you want your rig to behave correctly.