Time span for qualitative shifts of improvement

I have been discussing lately with my audiophile friends the following topic and we would be interested what you fellow GON members opinion would on this subject. What has been your experience regarding the approximate time span that it takes to replace different gear, wires, amps, pre-amps, etc., because of the evolution/revolution in older products or new pieces that the same manufacturer comes out with? The assumption is that your original piece was the best you could purchase and the new gear is a qualitative shift and not a lateral move. Please exclude either ego buying or the "flavor of the month club" in this inquiry. Do you think it takes a year, a couple of years or even longer for their to truly be a sonic difference that would mandate the expenditure of money to upgrade your system? As we all know new gear is always coming out, some people just get bored with their systems after a period of time and some fall for the hype about new stuff, but this question really revolves around real improvements and how long does it take for these qualitative shifts in your experience.