It depends on the size of your barrel. Smaller barrels have a larger surface to volume ratio than larger barrels, so there is a tendency for the impact of the barrel to happen quicker in small barrels.
I have a 2.5 gallon oak barrel that I keep full of bourbon so that I can "recharge" the bourbon character between batches. With such a small barrel, my best results have come when I leave the beer in there for 3-4 weeks and then I keg it. The non-barrel portion of the same batch is also kegged and used for blending if the oak/bourbon character is too high.
The nice part about having the barrel and non-barrel portions in kegs is that it is easy to pull a small sample of each and determine the ratio that gives me the desired level of barrel character. I have an "out-to-out" jumper hose that I use to move the beer between kegs, so the whole process can be done without introducing additional oxygen. I use a scale to weigh the beer as it is added to the receiving keg to get the ratios right. If you have bulk storage space, I recommend that you brew up a blending batch in case the barrel character is too high. If it ends up being just right, then you can put that second batch into the barrel when it is emptied.
A few other random thoughts about whiskey/bourbon/spirit barrels:
- No Sulfur Sticks (read Gordon Strong's book to learn the explosive truth)
- Have a plan for the barrel when you are ready to empty it because it shouldn't sit empty for a long period.
- The first use will have the strongest character from the booze, so it should be tasted reasonably frequently.
- It only seems like too much trouble until you taste the results.
