Generally speaking, the larger the barrel, the more time that will be required to sit in the barrel to pick up the barrel character. Hopefully, you'll have the beer ready while the barrel is still hydrated. If the barrel dries out, it will leak. You'll want to take a whiff when you get it to see if the whiskey is still noticeable. Get a wine thief to periodically sample to determine when the beer is ready. With a 15 gallon barrel, I'd probably check it first in about 3 weeks and decide from there.