Something similar to forgetting a cellphone happened when my dad and I went up to Lake Ontario. I don't remember what we forgot, but it was enough to turn back in a monsoon to get, with towels along the dash due to the leaking windscreen on the Rover. Still can't figure out why it does that...

I voted 30-45 minutes, but I think it would really matter on the distance I was traveling. If I was going to be gone for a few days, like on a lighthousing trip, then I would definitely. However, if it was just a quick errand, then probably it would be less. Circumstances would definitely have to be taken into account.

Interesting question, John!