Battle.net has word that the first Diablo III season competition is soon drawing to a close, offering an explanation of how the process of wrapping things up and rolling over items into players' regular inventories will play out. They do not offer a definitive end date for the season, saying it could conclude as early as February 3rd, and though this could shift, they will do their "utmost" to adhere to that date. They do not specify when season two for the action/RPG sequel will commence, but they do say there will be "a short period of time" between seasons to get your gear in order. This sums things up:

Note that all heroes, rewards, and progress will convert to the appropriate non-Seasonal game mode. Any items, gold, Paragon experience, Achievements, Artisan levels or recipes, Stash space, and Blood Shards earned on Normal Seasonal heroes with transfer to your Normal non-Seasonal heroes when the season is over. Similarly, anything you’ve earned on Hardcore Seasonal heroes will transfer to your Hardcore non-Season heroes instead.