I think OP is referring to the kiosk 24/7 Hertz locations when he says 24/7 and not Hertz On Demand which is the hourly car sharing program (like ZipCar).
This is the location I'm assuming OP is referring to:
http://www.hertz.com/rentacar/locati...sp&eoag=NYCC28
I've never rented from one but I assume the cars are just valeted by the parking garage and you need to give at least 3 hours notice for pick up. Seems to me you should be ok reserving for whatever time you like - if in doubt, call them?