Blizzard has announced that Diablo 3 Season 8 will end in all regions on December 30th. Season 9 will begin a week later on January 6th. On December 30th all seasonal characters will be transfered to the non-season character list with all progress and items retained.
At this time, the rewards for Season 9 remain unknown but we will keep you posted once they're detailed.
Once the Season ends, all Seasonal Leaderboards will be wiped and you will be unable to attain new Leaderboard ranks until the next Season begins. The results of the previous Season’s Leaderboards will remain available for viewing in-game. Once the new Season begins, a handy drop-down menu will allow you to swap between the current and previous Season records.
The full details of the transfer from Season to standard play can be seen on the Diablo 3 site.