if the ending is a dream it would be short
So assuming the ending is actually a dream he is clearly not still in limbo as that is somewhere where you create your landscape so he would have to create a airport and all his friends and choose to go get on the flight knowing its all fake which is very unlikely. the girl says find him and bring him back at the end. He then turns up with a gun which must mean logically shooting him and himself will kick them up at least one or all the levels otherwise it would be pointless to go after him if there was no way to bring him back
assuming it kicks them the next level up into a dream of them being on a plane the wait would be short before the sedation wears off and they are woken up.
dying in a dream was shown to not kill the dreamer so regardless of how long he would stay under eventually he will always wake up