My dweller is a level 24 Alexander Synder with high luck and medium everything else - he has 25 Stimpaks and 11 RadAways, he's already been out 9 hours but I wondered if he'd survive overnight for another 8-9h?
If he has some really awesome loot or too many caps, better recall him. But, if you don't mind a lvl 24 dweller dying, You can just leave him alone. Rule of thumb(suggested): Send a dweller while going to sleep. When you wake up, recall him/her if alive
Update: So I trained all stats of a dweller, gave her a railway gun and sent her to explore. Two-three times after doing this, she became L50. Revival costs 1000 caps. So I just send her with 20 or more Stimpaks and 10 Radaways and wait for two days. She brings a loot of 6-7k caps. And even if she dies, I just revive once and then recall.