If they just take out the fluid with a needle and "have a wait and see if it comes back attitude", then you can ski right away. I think I was 13-14 when I had it done (
the mind is going). It never hurt after. I started to wear shoes right away. It just never bothered me again. It left a stretch mark on my foot tho that is still there today.
The pain you feel now is from it pressing on the nerves in your foot. Once you get it aspirated (needle taking out fluid) then the pain should go away and you'll be back to normal.
If you would need surgery, you'd probably be off skiing for at least 3 weeks, possibly up to 8 weeks, depending on the severity.
You'll probably need to do something about it before skiing as those tight boots pressing on that are gonna kill ya. Hopefully they can just drain it and you'll never have to think about it again. That is usually the case in nearly all ganglions.