I have turned down extentions because it wouldn't leave enough break (I prefer 45 minutes to 1 hour inbetween), then been let down the instance I'm thinking of was someone who'd faffed for a while, then made the excuse he'd driven past my town to go for dinner with his mates. Had been a bit of a 'texter' which I gave the benefit of the doubt.
I do however work to set appointments, and prefer peace of mind to risking naffing someone off.