I've had several dwellers make it 5 days or more before running out of Stimpacks. If you have not played a Fallout game before, the S.P.E.C.I.A.L. Just like the main Fallout series, Fallout Shelter uses the S.P.E.C.I.A.L stat system. Additionally, when a Dweller levels up they are healed completely, so a SPECIAL 10 dweller w/ Endurance gear can often go a whole day or two before using her first Stimpack. The Most Important Stats for Exploring the Wasteland in Fallout Shelter. There are events for each of the stats in the wastes, so having everything maxed will give you the best benefit. Once they hit Level 50, switch to your best other gear and give a newbie the high Endurance gear. Then put your dweller in your best Endurance gear and send them out. If you really want to do well in the wastes, raise all 7 stats to 10 before reaching Level 2. It's also worth noting that at 11 Endurance Dwellers don't take radiation damage in the wastes. So a Dweller who has a 17 in endurance will gain the most HPs each time she levels. Whenever a dweller levels up they gain HP based on their Endurance, and this includes the gear they are wearing. If you can also equip them with +5 or +7 Endurance gear all the better.

However if you want a Wasteland Runner, you should at minimum raise Endurance to 10 before sending them out into the Wastes. Without using a Save Editor of some kind there is no way to see your Dwellers' Max HP stat.

Fallout shelter exploring stats