I provide a two hour window, but try to always run closer to the earlier time. I will call if I think I'm going to be more than 10 minutes late, so I agree with AJ's time frame. I think that is a reasonable time period that can be affected by traffic, weather, a needed gas stop, a wrogn turn, etc. I also try to call my later scheduled customers to be sure they are around, so I don't waste a trip.
Seems that customers have become much more demanding. I think that is a by-product of our 'right here-right now' society where many things are available instantly (ie: instant movie watch, instant information via the internet, 24 hour anything-you-can-think-of drive-thrus!)
What we need is a transporter, so we can be there RIGHT NOW! Beam me up Scotty; there's no intelligent life here!!