I always given them a set time/date to confirm our booking by. It is their responsibility to get in touch with me and if they miss the deadline, the date is cancelled, no exceptions.
Regarding being late, I would probably offer a 5-10 minute grace period but if they're later than this, particularly if they haven't let me know they're running late, I would cancel. (However this said, I always go to them, so the onus is on me to be on time).