How new is the barrel? How many batches of beer have gone through it? If it is new, and 5 gallons, it will impart a lot if oak flavor. It won't need more than 2-3 weeks and that might be too much. A 5 gallon has more surface area than a full size barrel. Every time you use it you will get less oak flavor. They are usually good for 4 or 5 batches. After that you will have to add oak chips, cubes, or spirals.