We don't last very long either, probably 5 minutes tops. I do make sure to put him on his belly a few times a day though, 5 minutes here or there.
Now finn won't stay on his belly on the floor, he rolls over immediately, so i'll put him on his belly on his countoured changing pad and put a toy in front of him and the sides keep him from rolling and I think the change of atmosphere might help. He tolerates it much better than being on the floor.