The timers vary. But at this time it's believed that once lit, 48 hours. Hope this helps.

Not sure how long the lighthouse lasts, but the island will eventually sink and reset. Also not sure how many islands there are yet. And there are multiple islands with the same mobs on them. We have a lighthouse at a flosar island and I managed to find a 2nd island with the same mobs. Still searching for the magical rachni island Oh Rachnis, a memorial to you is not enough, we are so sorry we made you hide by hopping on you daily. Hopefully you'll come back out to play someday?

thanks everyone, this helps!

Rachnis are extinct. From what I heard from Rus server, they might bring them back for a limited 2 hr event like hero mobs.

I mean island(-s) sink every 48 hours so no longer than that. In that regard Leanne is pretty much correct.