Normally a transportation company picks up between 3 and 3 1/2 hours before a domestic flight.
It used to be 3 hours but now that the service is not as widely used I'm finding it can be a bit earlier. We had a 10:10AM flight a few weeks ago and our pickup time was 6:35AM so 3 hours and 25 mins before our flight. Our experience was with Mears Connect.