How long should I leave my poor soul out there to wander the wastelands? I see that his SPECIAL S & E skills have a blue increase bar - but the status time just keeps going up instead of counting down. When should I call him back - or will he come back on his own?
Explorers will die on their own, you need to call them back before they run out of health and supplies. On the positive side, they don't take any more HP or radiation damage while they are returning home.
I personally tend to recall my explorers in these situations:
1) They found a sweet piece of loot that I want to get back safe.
2) I'm about to do something IRL where I won't be able to check on their progress and they are unlikely to survive that time. This is usually at night before I go to bed.
3) They have run out of stimpaks or radaways and are at significant risk of dying. This can mean a lower level explorer with nearly full health (because they don't have much HP) or a higher level one with a good percentage of damage/radiation poisoning.
The overall idea is to balance the risk/expense of them dying versus getting better loot for exploring for longer. Most of the time I recall them for reason (2) as I like to keep a very close eye on them.
Dwellers will never come back on their own and will keep fighting in the wastelands until they either die or you recall them.
The longer they remain outside, the tougher enemies they'll encounter, and the better rewards they'll find.
The moment you call them back, they'll stop having encounters. Because of that, you should be safe if you call your dweller before he's dead, unless of course you want him dead.
Recall your explorer when you will no longer be able to micromanage him and watch his health. I prefer to do this right before bed, since he will take no more damage on the way home. For example, if your explorer has been going strong for 7 hours but you need to head to sleep, it might be safer to recall them instead of risking death.