According to a tweet from @EAAccess that was published yesterday, August 3rd, the EA Access Trial period will begin on August 20th and will last for 10 gameplay hours or for 5 days until Madden 16 releases on August 25th.
EA Access always releases trials 5 days prior to a game’s release, but Madden 15 only gave 5 hours worth of gameplay access. It was widely assumed that Madden 16 would also receive 5 hours of gameplay, but EA increased the time to 10 hours.
You can become a member of EA Access for $4.99 per month if you are on Xbox One. There is no minimum month agreement, so you can just subscribe for 1 month to get the early release of Madden if you chose. If you chose to buy digitally from EA, then EA Access will also give you a 10% discount on Madden 16, thus paying for itself in the process. So it really is a win-win for gamers.
There is no official word yet on a possible demo of Madden 16.