As mentioned, it'll depend on when it's made obsolete. If Nvidia waits until 16nm to put anything else out, it could be up to a year or so before it's no longer the current generation. Price cuts along the way will happen, but no much I'd imagine.
If Nvidia does decide to release a refreshed Maxwell line to fill the gap, then it'll drop sooner as I'm sure they'd have some kind of replacement. However, without being a full blown GM200, I don't see how they can squeeze any more performance out of that size of a die at 28nm. From what I read about the current state of TSMC's 28nm is the GM200 is pushing it as it is, so I can't see them going any larger. It's already a huge die, and I kinda did a double take when I pulled off the blower from my 980 Ti to put on the waterblock.