I still have a pair of speakers that I built in 1978, though I rebuilt them again mid-80s. They are still going strong and are still in good condition.
Though I confess I bought some new speakers in about Sept 2009. I'm considering replacing those, but it will be a few years before that happens. I bought a new amp and that makes me want new speakers, but that is hard to justify when the speakers I have sound fine.
Back when I was young and working my way through turntables, speakers, and amps, it was about every couple years. Though back then it was easier to sell equipment used. Then I found a system I was satisfied with, and that lasted me for decades. But, the amp got old and need to be replaced. I went through a couple amps, until I settled in on what I have now.
My old amp was on my computer, when I bought a new amp for my new speakers, the other not quite so old amp got moved to my computer, and the really old amp got retired.
Even more so now, it is hard to get to hear a lot of amps and speakers. Unless you live in a very large city, you have few options for auditions. Most of the equipment I bought, I bought on faith, meaning unheard. I went by the reputation of the manufacturer and have done pretty well over the years.