I haven't seen a limit on how many items they can pick up yet. I had Bittercup who came with crazy high stats out for 34 hours and she just kept collecting, mostly junk, until I called her back. She was level 41 and had Luck +5 and Perception +7 which helped her find tons of gear, but only like three or four high level items, a gauss rifle, a upgraded battle armor, upgraded merc armor, and a combat shotgun. The biggest benefit to anything over six hours is the XP they get for being out that long. After 24 hours everything she killed was >1000XP, Mirelurk Hunters, Feral Ghoul Reaver,... Other than that I don't see much reason to have them out that long, just send two people out every hour and call two back in leaving them out for 6-8 hours. Add time as you start sending more people. When the vault is 100+ people you'll want to send out like 15-20 or you'll have to much to do in the vault.
EDIT: Also, one thing that might help some people is building two sets of elevators. There is space for eight rooms and two elevators on every level (except the top). I like to have the three rooms on the left, then an elevator, three rooms in the middle, then an elevator, then two rooms on the right.
On the right I only use it for living quarters and storage rooms while all the other rooms can be three spaces wide.