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.)
But apart from the expansion, what does it mean for PvP?
Well, it's good news for world PvP, or at least I hope it is. A new season, as Brian said, with new rewards and new gear, means an increase in PvP item levels. If previous patterns are followed, honor gear would become 522, and conquest gear would become something like 548. This would put PvP geared players on a more even footing with their PvE geared targets in world PvP, and knowing the current issues the devs could increase the PvP Power on new items to compensate for the item level difference.
I'm more concerned for arena, though. My main worry is that the devs will decide that now is the time to bring in a planned change I really dislike -- moving the start of the dampening debuff to five minutes. Why do I dislike it? Yes, it's certainly related to how I main healers, and I hate the feeling of becoming less and less useful and good at my job regardless of how well I play. I dislike the change in power, my character becoming less and less powerful through a match. There are so many better ways I can think of to fix the issue it attempts to address -- regen changes being top of the list -- but none of them are likely at this point. They're too big and involved.
Another objection I have to it is that it was supposed to be a match-ender, replacing The Crowd Chose You. At the moment, with dampening at 10 minutes, it's rare to even see it. With it at 5 minutes, you'd see it in most matches. It's not ending a match that's running long at 5 minutes, it's kicking in early.
But that's personal opinion. Maybe this will be a good time for some further balance fixes, too, as some are still needed. Base resilience may increase. And we'll have to wait and see what happens with the interaction of what will, presumably, be Mists' last season, and the squish of items and abilities which is currently slated for the Warlords pre-patch. My prediction is that the season will end with the pre-patch, just like in the Cataclysm to Mists changeover. It'd make the most sense after all.