It takes over a year of not being powered for the capacitors to begin to de-form, but you have to go by the date they were manufactured or taken out of service, not the date they were bought. If they have been unpowered for 2 years of more, you have to use the reforming procedure, between 1 and two years it’s iffy, so just never go more than a year and you are fine.
So if you have spare drives on the shelf, one simple way to deal with this as a preventative measure it is to take them off of the shelf once per year, hook them up to rated line power on a bench (with all due safety), let them sit there like that for a couple of hours then put them back on the shelf for another year. I make up a sticker for the box that charts the PM dates.
Bonitron sells a pre-packaged capacitor reformer. It’s spendy, but it’s hard to find the parts to do it on your own too. I bought one of the Bonitron units and I made one of my own with a variac, I think I spent maybe 20% more for the Bonitron when all was said and done, but if I value my time correctly, I lost money on the DIY version, plus the Bonitron comes with a nice roller case.
I take that one around to service calls, it’s paid for itself in under a year.