It seems that by going on the raft, you are assuming most of the risk. The likelihood of dying on the raft, compared to the island, is much greater (but we have the benefit of previews that show the struggle with "the others" next week, so you can't take that into account). There is the risk of weather, starvation, sharks, etc. I would think the most you could survive would be 1-1.5 months. But if you do survive and get picked up, the search parties should locate the island to save everyone else.
But, going on the raft is necessary, IMO. The search parties have probably long ceased and the only way of being spotted would be on the raft.