How long before things get bad?

Probably about an hour into the film.

Two and a Half hours seems a little long... I mean, it'd make sense, a lot has to be made up for in the decade inbetween films, but, thats a lot.

In any case, They should have the basic reasons for the cast being at the island established maybe 10-20 minutes in. With them introducing the island from 30 minutes to 40 minutes in, with the main problem, or foreshadowing happening right after that, With the main problem being established around an hour in, a near revival or solution around an hour and a half in, with that failing, followed by the movie increasing in action until it hits the climax and falls to a close. 

^Most likely scenario