IDK the official answer (wasn't trying to test the limits but that's what happened) but my guess is 15 minutes? I meant to check in to Kona when our boat from MK was leaving the GF for the Poly last week. I forgot (like I did most times re: LL booking without an alarm set), wrapped up in the gorgeous weather, views, some Poly nostalgia, having all four of us in the same place at the same time. By the time we docked at the Poly it was 17 minutes past our ADR and app said online check-in was unavailable, please see the CM. He was trying to be funny but came off rude, disbelieving that the app prohibited us from online check-in, but it wasn't a thing. Kona didn't fill up until we'd been seated for 10-15 minutes, so we hadn't held anyone up.