I was given a 3 hour window 11.00 am and 2.00 pm. I called in at 1.00 pm to check the status. Then instead of sending the plumber within the window, they called me to say they are running late and will be there between 2.00 pm and 3.00 pm. I take this as a sign reflective of professionalism.
A 3-hour window is long enough to be met 100% of the time for any business.