According to a Twitter post by Blizzard's Brian Holinka, World of Warcraft players can expect a new PvP season to start and finish prior to the launch of the Warlords of Draenor expansion. The good folks at WoWInsider.com have speculated that, at the earliest, the current season would end in February and a new season towards the end of June.
The speculation has it that the new PvP season would end and mark the release of WoD.
Can we therefore predict the length of season 15? Not really, but we can confidently say it's likely to be over 20 weeks. Bearing in mind that we're 16 weeks into season 14, we may well have another 4+ weeks to go, meaning that season 14 will end at best February 4, so a 20-week duration would put season 15's end on July 1. This would be highly optimistic. (Edit: And as commenters add, the season end has previously marked the pre-patch, not the expansion itself.)
Think they're right? Let us know what you think in the comments.