You tip when the car is returned to you, although if you're a regular and your schedule is such that the folks that take the car when you arrive always lose out, I would sometimes give them a nice tip too.

How much depends on the location/region, class of hotel, etc. as well as whether the service is good. I tip more in bad weather, especially if snow and ice were removed from the vehicle. A few places might clean the windshield or even wash the car for you (or otherwise go above and beyond), which should be rewarded.