I have a few different amps, the oldest being a Denon POA-2400 from 1989. None of them have needed re-capping yet. Even some older amps that I have given or sold to friends are still functioning fine. Of course there are many variables, how hot the equipment gets, how often, and how long it gets used, etc.
I also have some speakers from 1989. I was convinced to rebuild the crossovers on them in 2006,. I was told the values would be way out of spec at 17 years old (caps and resisters), and that modern polypropylene caps sound much better than the original electrolytics from the factory. I couldn't tell any difference when I was done, so I measured all the caps and resistors. They were all still within spec.
Fix it when it breaks.