Without bags, maybe $1-2 each time you take the car out. Not when you bring it back again. Or if you can find out if they pool tips (ask the front desk), you could just leave a big tip at the end.
I usually do $2 every time I have them get the car. And I sometimes throw a few extra bucks to a certain valet at the end of my stay if I had the same one the whole time.
My H and I always disagree about this too. I say $2-3 and he says $5 min. Ughhhh I hate paying for valet. It's like a 5 min service and I would rather jut park my own car.
I'd do a couple bucks each time they bring the car around, since you're probably paying for parking. If the valet is complimentary I'd bump it to $5. Then I add $1 per bag if they're loading/unloading.