From my experience, 30 mins has never been a problem with any rental agency. But that obviously counts from when you actually took over the car, not when your rental pick up time was supposed to be. Not aware of any places that would charge excess time by the hour, but always by an extra day once you're past the grace period.

When I worked for a smallish outstation of a major agency, we would often overlook later returns (time of parking garage entrance ticket) if the car was returned during the times when we were closed. Fighting over an hour here or there when we weren't going to use the car seemed hardly worth it. YMMV on that one.