How long is too long in the Wasteland?

I've read the answers and comments from this question.

I know:

• encounters get more difficult the longer you stay out.
• loot gets better the longer you stay out.
• there is a special encounter at 2.5 days, with a chance for ledgendary gear.
• you can only send 25 stimpaks/rad aways with any one dweller
• a dweller who collects 100 items (weapons/armour) will return to the vault on his/her own.

Is there a point where it is more valuable to return (and take another run at the National Guard Depot) instead of keep going until your supplies run out or your inventory fills?

Test case: I have an explorer who is over 3 days out, but still has 13 stimpaks.