Upvote:2
It depends on the airline and you will have to contact them to find out exactly.
You should ask about their policies regarding two things:
Schedule change window. The airline should have a policy that gives the maximum time they can reschedule the flight before offering re-accommodation or a refund. American Airlines is 4 hours for example if I recall correctly.
Trip in vain. This means that a schedule change or delay would make the trip meaningless such as missing a meeting or cruise ship for example because of the different departure time. Basically, there's no point in flying.