If you are trying to metagame this, there are some statistics that can be found on the web to help.
A basic (not 100% accurate) summary is:
About every 15 minutes spent in the wasteland will give you a chance at some caps.
Around every hour spent in the wasteland will give you a chance at caps, a normal item, a rare item or a legendary item. The longer you survive in the wasteland, the better your chances of a reward other than caps/normal items. Dying and being resurrected seems to reset your odds back to when you first left the vault.
There are special encounters that occur at fixed times. The first of these encounters will normally occur soon after leaving the vault. The second about a half hour later. Somewhere around an hour and 15 minutes, you will have your third encounter. At about the 2 and a half hour mark another encounter. At the 5 hour mark another encounter. Then 10 hours, then 15 hours, then 20 hours, then 30 hours, then 40 hours, then 60 hours. Again the longer you are out, the better chance you have for a rare or a legendary.
Each of the timed events will test a different SPECIAL attribute which you must pass for a chance at loot. As an example, raiders will test your dweller's agility, NG Depot will test Perception, merchant will test Charisma, etc.
There appears to be at least one known timed event after the National Guard Depot (60 hour mark) but the Depot is also the point of diminishing returns. Chances are excellent that the best trained, best equipped and highest level dweller will be killed before reaching the 75 hour event, so it is best to start them on the long journey home after they reach the Depot.
Additionally, it should be noted that the longer your dweller is outside, the nastier the 15 minute events become. This seems to cap with Behemoths. This is NOT reset if your dweller dies and is resurrected.