I have an old TenTec Argosy II that hasn’t been powered up for 20 years.
How long can electronics go unpowered before I have to worry about the electrolytic needed a slow power up with a Variac?
There is no set time period. Play it safe, bring it up slowly and be done with it.
You can also put a ~100w incandescent lamp in series with the AC line (or better yet, a clamp-on AC current meter) as you bring up the voltage. If the capacitors are not bad, then the lamp should dim as the capacitors charge.