I second Deb's answer but want to add:
Was the pickup time discussed prior to the beginning of the stay? If the client has informed me ahead of time that, for example, their flight gets in after 6pm and they won't be able to pick up until late, in the name of repeat business I am unlikely to charge extra.
I do quite a bit of communicating with clients during the stay and, if my gut says they will be late picking up, I might let them know I have 'an appointment scheduled' to encourage them to be on time.
If they have not planned ahead for a late pickup and I have sat there for several hours, steaming, then I will likely extend their stay through Rover (and do some deep breathing).