I just do mine around 3pm because it is the hottest part of the day and I can put them in direct sunlight to help keep the water warm also. I am thinking of doing it for 20 minutes in the morning as well though...would have to be after they eat. They wouldn't be happy if they had to wait any longer for breakfast.
Well, Myrtle's old enough that I am only soaking her once a day. I do it in the bathtub right now because I don't have anywhere else, so sunlight doesn't matter. And the time is the best one I cam work into my schedule.