The "leakage" that DW mentioned is internal current flowing through the capacitor,
not physical leakage with liquid coming out and forming a puddle on the floor.
I concur with the every three-to-six month strategy.
Powering up a pack for perhaps 15 minutes should re-form the caps and keep them in good shape.
More often would be better than less often. Capacitor degradation and failure are really not predictable.
If a strobe has been unused for several years, or is of unknown condition,
there's a safe way to bring it up (if that's possible), to wit:
Pick a convenient time of day; doesn't matter what as long as you're available every day for a week.
I'll use noon in the example (don't fire the flash until the end of the week).
At noon on the first day, power the pack up for ONE minute, then turn it off and let it sit.
The next day at the same time, turn it on for TWO minutes, then off.
Repeat this on successive days, doubling the time each day, i.e. 3rd day 4 minutes, 4th day 8 minutes, etc.
At the end of a week you'll have the pack on for an hour. At the end of the hour you can fire it, and use it as desired.