At the two different places I was at, we had a 1 hour and this last one was 45 minutes. Clock starts at ETD (and if youre on the CJ, the 10/24 hour limit was a real factor), so if 45 minutes/1 hour went by, with no calls, no answer from the passengers, or the brokers, it was go time. We would replan, refile, and off we went. Contract will dictate, but no calls no shows were non refundable. Ironclad and bulletproof contract helps with those.
Most of the time people were 30 minutes early, and I would never say no to departing early unless the previous days events prevented an early departure.