An estimate based on the GPS ETA with suitable subtraction amendment for distance to be flown. However, if it looks like you are going to be well out, for example due to high local winds on departure but not expected for whole flight, one can always qualify the departure report with "...will update at top of climb."
I should add that an accurate ETA is as long as a piece of string for the multitude of factors that enter into the equation. I used to use a company generated flight planner that was remarkably accurate for the aircraft that we flew. Time and time again I was quite amazed at how accurate it was at generating the ETI for that first leg, probably more so that the GPS with suitable amendment. Regrettably we can no longer use it due to the almighty upheaval in waypoint names by Airservices (God rot their cotton socks for that).