when do you replace capacitors?

As for a long time use/non use, I would say the time is time. used or not, for caps. Capacitors actually LIKE to beused some. Not too hot, not too seldom.
So the twenty years is used a lot or not much.
Some discussion is given to theory about high temps will cause premature aging of caps in hot running electronics, and that they dry out faster. But little real evidence to show that is really true IMO.
Some caps just leak or die no matter what.
The one bad thing is a LONG time and never used, is very bad for capacitors when they get turned back on for the first time. KaBOOM!
A super no cost way to reform them (what the process of getting them back after yearss of no use): plug the unit in. Turn on for the briefest possible time fast switch on/ off under one second. Wait ten minutes. Turn on again for only two seconds or so, then off, (don't stretch out the time) leave off for one half hour. Turn on for five seconds, then off. leave off for several hours to overnight. Then it should be good to go.
Cheaper than other ways to safely reform caps. Works too.

Do ALL the caps at one time when you decide to have them done.
Twenty years is a safe period.
It might not be neccessary to have all of them done then, but better that, if you really like the amp, then have one blow up on you suddenly.
I you want to do them early, no problem. Especially if the company is still able to do it for you, instead of some third party.
Think of it this way: if you do them now, that gives you 20 MORE years of service from now. And in four years, heaven forbid, maybe they will not be able to redo them for you.

Variac is the porper way to restore a cap. If your electronics item is worth a lot of money, and you have the money to spend getting it variac'ed. Do it. Otherwise the turn on style will do "in a pinch" or if you are just too cheap to get it done right. It is a 'dirty' method, but works. (than means it is not approved by anyone who actually does this stuff for a living, because it cuts into the bottom line)

So, the equipment is going to "explode in boiling fire"? Or is it that the cost of replacing a blown cap is less than renting a variac?
I can understand if the item is a bit of medical equipment attached to your heart pump. but usually old stuff that was stored for ten years is not the usual Ferrarri material.
I would not be afraid to use the cheap and dirty method on ANY product i owned.
Any yeah, if the product means a LOT to you, and it is really valuable, then splurge if it makes you feel better. But the item is not going to explode with the turn on method any more than with the variac.
This is like do you take your car to the dealer? or a local guy who you know.

Perhaps I just like to live dangerously?
I deliberately bought a SinglePower MPX3 because the notorious 'exploding with brimestone and boiling fire, killer/blinding headphone amp was just too good a conversation piece to pass on. Aparently some dude over at headfi got in a tizzy over his product being shoddily made and declaired all SinglePower products to be timebombs waiting to maim and kill the owners, and everyone needed to stop using them immediately, or risk immolation.
I have used my SinglePower now with it on the higher bias setting and so far have not been killed. (though some wish I was dead..sigh.)
Or is it I just am practical?
I stand up for cheap HiFi. and the turn on is fine by me: cheap.

Thanks Minori, I was unaware that the silicone grease can degraded over time.