How long do I have to wait to give my 1 year old benadryl after I have given tylenol?

I'm waiting to hear back from the on call nurse. It's been two hours though and still haven't heard from them.

About how long should I wait to give my son benadryl? I feel like an idiot for not asking at his appointment. 🤦‍♀️

*The benadryl is prescribed by his doctor for his crazy allergies. I know its uncommon.*