It depends. Some of the Sonics around here have drive thru windows. So no tip there! The ones that do not have a window, have a drive up line, and the "hop" walks out the door about 5 feet to your car window. I usually give them the "change", never less than about 50¢ though. (If the "change" isn't that much, I throw in enough). If I park at a "booth", then I tip a dollar or so. If I am in the drive up line, but the hop brings out my stuff before the other cars in front of me (which they will if your order is ready first, like if you just order a drink and the cars in front ordered food)... then they probably get a $1.00.
It is not the same as restaurant service, they aren't cleaning up after me, or bringing me extras or refilling drinks and checking on us. They are bringing me a bag and a cup (or 2)... kinda like the bag boy at the grocery store. They are paid minimum wage, not "server" wages ($2.10/hr).