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.
Speakers; 3-5 years

Cd Players 2-3 years

Amplifiers 10-20 years

Preamplifiers 10-20 years

Turntables 30 years

Surround Processors <1year

Hope that is what you were looking for;
In my book, the average time span would be every two week pay period. It seems that time is not necessarily the measure for qualitative shifts of improvement rather it is the amount of money one has to spend for the improvements.

One could take an inexpensive CD player and replace all the paper caps and cheap wiring and such with higher quality parts and the sound will be completely different, an improvement.

Yes, current advances in technology, which I believe you are referring to as far as time span, does improve some aspects of the sound. But how would one explain the popularity of tube gear if solid state is a technological advancement? Is the new DLP, LCD, LCoS technology for video better when the old CRT picture looks superior?

It actually would depend on the listener and viewer, many people are satisfied with the mass market sound of Sony and Bose and actually believe this is new technology (because it is the latest and greatest model they have seen). Some of my friends (non-audiophiles) and relatives (wife and parents), cannot believe the money I put into my system. To each their own. If you can afford it, why not go for it!

This a good discussion but one that could last for years or at least to the next qualitative shift of improvement.