I have a HPF magnetic RS 1 lamp ballast (Advance) and I was wondering if since the capacitor is going to fail one day, would it be better to use it now, or keep it in storage for another day. I don't know much about ballasts so any input is appreciated!

I'm no expert but my impression is that the film and oil type capacitors used in magnetic ballasts have a long shelf life, especially compared to electrolytic caps frequently employed in electronics.  So if kept in storage there's a good chance it will still be in good working order years later.  That being said, is the ballast anything special?  Even in use, the ballast may very well last for years, possibly decades.  If you're going to have it running 24/7 or anything close to it, wear may be a real issue, but if you're not going to have to on more than a few hours a day on average, there's a decent chance you'll never see it fail.  In my limited experience, ballast capacitor failure is unpredictable.  I've had used ballasts where the caps are significantly degrades, and others that apparently have just as much use which haven't deteriorated at all.

For all plastic film types, the clock is tcking only when power is applied

Oil types depend to great extent on the quality of sealing of the case. Seals that leak oil do lso let in diffusion of moisture, which in small quantities man embed itself into the oil and start a slowly acting destruction mechanism

It sometimes may appear, that powering the ballast after long non powered period may initiate a failure. This indeed may be the case if the conditions have set up during the time it was not powered, like significant moisture accumulation. (With power regularly applied, the damage could possibly be stretched to small steps over a long interval, so not happening all at once with the resulting excessive power dissipation and consequent failure)

However, it is the running of the ballast that sets up the conditions for the moisture to get in in the 1st place : The heat of the ballast when working does eventually wear the sealing of the case (drying gaskets etc), and makes the oil expand so push the seals, and contract when switched off and cooling down

I too think that a low use application will probably be an alright choice. Storage without use is as good for plastic film caps, may be as good for NOS oil filled capacitors, but maybe less so for oil capacitors that already did many hours of continuous work and have significant wear on their seals

That's good to know. It's an Advance single lamp HPF ballast and someone said it was uncommon so I wanted to preserve it.

I'm wondering if the older Advance ballast in my grandparents basement that suddenly failed failed because of the humidity because it wasn't used very much but the basement was extremely humid. Me and Alex were calling when it suddenly failed (the lamps kind of did a reverse rapid start) and it went from working perfectly to completely out in about 3 minutes.