when do you replace capacitors?

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)

Not because it cuts into the bottom line, because it's much safer.

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.

A qualified technician can apply a variac and reform the caps over time if they have been sitting for awhile. However, the tech must first inspect the caps to see if leakage has occured. Caps can be different. There are paper caps, oil filled caps, etc. So, they will age and fail differently. This is why the best caps are stupidly expensive vs. cheaper caps. Personally, for a device that has sat unused for many years, I would apply a variac and slowly bring up the voltage over a period of days to reform the caps and see if some other component has failed. If everything is okay, I would not change the caps unless they are leaking or won't reform. Also, another area people absolutely don't know about or forget is that power transistors typically use silicon transistor heat sink grease between the transistor insulator and the heat sinks. This grease definitely hardens and fails over time and for older amps is the main reason (other than the owner switching interconnect or speaker cables without turning it off first) why amps blow. For a really nice older amp, I typically will test all the pre-driver and output driver transitors, see if they are within spec and if not replace them (or replace them on general principle with newer more linear transistors) and absolutely replace the silicon grease. Of course I am an Electrical Engineer and also work on amps/components and upgrade them all the time. However, a good technician can do this also. Again, if you manage to grab a Mark Levinson ML3, 23.5, Bedini, etc. amps absolutely check the caps and change the silicon grease. But, if the caps reform and aren't leaking, I wouln't replace them just yet.

enjoy.

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