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.