Can you tell me when does the drowsy feeling of benadryl (diphenhydramine) go away?

A few hours: Benadryl (diphenhydramine) is a short-acting medication, so i would expect it to go away within 4-6 hours in most people, although some people are just in general a lot more sensitive to it and experience drowsiness for a longer period. If you're having problems, you might get tablets or liquid and try taking half of what you usually take (half a tab or 1 tsp of liquid is 12.5 mg instead of 25 mg in a whole tab/cap.