The image above is taken from MacRumors' Buyer's Guide which has always been a very reliable source of reference for when you should and shouldn't buy Apple hardware based on Apple's usual update cycle.
But everything about Apple lately has suggested that they are moving away from a usual update schedule. Like, very unusual. Every machine Apple makes with the exception of the super-skinny MacBook and the iMac, has not been updated in over a year.
Not too long ago, I caved in to buying a new MacBook Pro with Retina display because I just could no longer wait for Apple to push out an update. If I had kept waiting, I'd still be waiting to this very day (and probably much longer).