Lego put up a bunch of the 2015 sets on clearance for their Black Friday deals so it definitely is looking like they're already phasing them out (may be worth noting, Pohatu is marked down at the moment on the online shop and that should apply in-store as well). I haven't been to a Lego store myself in over half a year I think so I don't really know what's going on there too much personally, though. It may depend on the size of the store too, mine is pretty tiny so when it's time to get rid of old stock they try to do it as fast as possible.

The first sets to go will be the winter sets—Toa, Protectors, and Lord of Skull Spiders. That said, they shouldn't disappear entirely right away, and in fact now is probably going to be when you'll find the best deals on the winter sets, since retailers will be putting them on clearance to make way for the new Toa. Be on alert for deals and if you REALLY want the other sets, don't hesitate longer than you have to.

The Skull Villains and Ekimu will probably remain available for several more months, since most sets generally stick around for a good year or so after their release.

And of course, even if you miss out, there will probably still be resellers hawking the sets for quite a while, and while they will go up in price once they're not available elsewhere, that tends to be a very gradual process. So don't let yourself get TOO stressed out if one of the sets disappears before you get to it.