I think it depends on the time of day. We returned our car just before a shuttle was going to the terminals, so we didn't wait at all. It took about 10 minutes to get to our terminal. I'd suggest asking Hertz what time you should return your car so you can make your flight.
I'd suggest giving yourself 30 minutes to get to the terminal. That way if you have to wait to check in the car and wait for the shuttle, you'll have given yourself enough time.