Holdover times have usually a time range. Something like 25 minutes to 45 minutes. So for the first 25 minutes at the given conditions, protection is assured. Once those 25 minutes are over the flight crew needs to ascertain that the anti-ice protection is still holding, that can be done via a visual inspection. And yes, it is first drop to start of take off roll, during the take off rall the anti-icing fluid will flow mostly off the wings carrying with it the contamination and leaving the wing clean. Once in the air the aircraft installed ani-ice equipment has to take over if needed.