I used to be really curious about diphenhydramine because I just wanted a medicine that would always be there so I could always sleep my problems away. But in reality I just always felt so sick from the high doses I needed to fall asleep that I wished I was dead....
1. 30 minutes to kick in depending on contents of stomach. Like any drug it takes time to be absorbed through the stomach lining, but it does start working pretty fast. Closer to an hour to feel the full effects
2. Tolerance builds up quickly but also decreases quickly once you stop taking it.
3. It's possible to take enough to kill yourself, although it takes a lot and it would be a very unpleasant experience. I tried it once with like a whole box and I just felt so sick and slept for literally like 3 days. It's definitely not a sure way to go and it would cause suffering as how it would ultimately kill you would be by damaging your organs (liver/kidneys). It would not be a quick "die in your sleep" kind of death. --- If that's what you're looking for, I suggest you try to seek help and talk about what's really going on that's making you feel that way.