There is a very up-to-date website that tells you when the last Apple products were released, how often they are released and when they are predicted to update again. It’s called MacRumors Buyers Guide and you can access it here:

This guide covers the iPhone, iPod touch, iPad, iPad mini, MacBook Pro, Retina MacBook Pro, MacBook Air, Mac mini, iMac, Mac Pro, iPod nano, iPod shuffle, iPod classic, displays and the Apple TV.