In brief, players will always have access to the Dyadic Drift. That island is always sitting there, and if players don’t like their other options they can always hunt the monsters on that island. (Just don’t call in monster hunting.) However, the other two islands available are each on a staggered two-week rotation. So you will have access to a new pair of islands (with new enemies) every week, and each island has a two-week lifespan once it drifts into range. Or, to put it more simply, you should get to your favorite island and get hunting sooner rather than later.
Dauntless is not static. Islands rotate in and out of the game’s play area constantly. If you’re unfamiliar with the game’s way of running this, it can be a little… well, daunting. Fortunately, the official site explains the whole process with its latest entry explaining the island rotation.
Source: Official Site