First off, I am quite sure that no one has ever compiled any meaningful, broadly applicable statistics about this. Second, just a brief look reveals that for a fair number it is an extremely short period of time, and for a different but equally fair number it is an extremely long period of time. The variation is so great that the only logical conclusion is that it is a very personal thing. Meaning that the "average" will tell you almost nothing about yourself.
This is a bit like asking what is the "average" amount that people pay for a liveaboard/cruising boat. It doesn't take more than a few minutes of perusing this forum to see that there are a bunch of people who spend upwards of millions of dollars on their boats, and there are just as many on the other end who get by on very little. Taking an average of those widely diverse data points will tell you nothing useful about what YOU will spend on YOUR boat.
I would end by saying, simply asking this question indicates that you have doubts. If that's the case, then don't be in a big rush to sell things that you won't be able to easily replace when the time comes. Keeping a TV makes no sense as it will be obsolete in a few years anyway. Keeping a valuable, antique dining room set, though, that's a different matter.