Benadryl takes an hour which is 60 minutes to kick in and is quickly absorbed after oral administration and peak effects are reached within one hour.
The Benadryl after it kicks in should last in your system for around 5 to 6 hours depending on the dosage.
You can also use Benadryl at night to sleep and the recommended dosage of Benadryl at night time for sleep is 50 MG.
But if you need more then you can do so but don't take anymore than 100 MG of Benadryl for sleep or for during the day for allergies unless your doctor says it's okay.
Also never give babies under 2 years old any Benadryl unless a doctor says it's okay and tells you the recommended dosage.