A golf Futures bet is simply a wager on a golf event that will happen at a later date.
You can wager on a golf Futures market at anytime, but the sportsbooks will adjust the odds over the duration of the season depending on results, the course and other relevant news.
In general, an early Futures wager will offer the bettor with bigger odds, but come with greater risk (the uncertainty of how events will unfold before the wager is settled).