I spent my time finding out where these guys go the Sgt. died on account of being jumped by legion assassins, and this is how long it took
(Note: it maybe a little off)
Split 1: 00:02:00 Sgt. dies
Split 2: 00:09:04 we reach a PG camp
Split 3: 00:12:13 we reach hidden valley
Split 4: 00:15:44 we begin our trek through scorpion gulch( God do I hate that place)
Split 5: 00:22:43 we make it to HELIOS ONE
Split 6: 00:37:01 And we arrive at Camp Forlorn Hope where he stays

Hope this help with people who said he disappeared.