I don't know the exact timing but I know there are many months between filming the show and the finale in the studio.
Like maybe 10 months. You'll also notice how filled out they all are. They leave the island after 40 days of practically starving and then they have all those months to come back to full weight.
It is a game so there is a lot of artificial stuff going on and the show is edited a lot so do we get a fully accurate picture of what's happening? - probably not. It's kind of fun to watch tho and something I'd rather watch than something like the bachelor/bachelorette nonsense.