It took me 5hrs in good conditions. I parked at Pelter Bridge and went up by Nab Scar and did the clockwise route, leaving the return route to Ambleside at Low Sweden Bridge to drop to the path back to Rydal House.
First light is 0750 on the shortest day, sunset is 1548 on the shortest day and dark is at 1630. In theory you have time if visibility is good, if there is no snow and you move quick. Anticlockwise would be the best option and cutting up the side of Scandale Beck from Rydal to gain the upward route will save time. This way you also have a drop off option and the navigation towards the end is easier.