Depends on what you are going for. If you want to just add a little bit of whisky/oak character to your beers, you can just add fermented beer into the barrel and extract some of those tasty flavors. Depending on how long ago it was emptied, you may have to clean and sanitize the barrel. I believe there are sulpher sticks you can burn in them to kill off most of the nasties, and a little starsan probably wouldn't hurt, but I'd read up on it before jumping in. As for how long, 10-gallon barrels are going to have a lot of surface contact with your beer, so the flavor should develop fairly quickly. I'd plan on testing it out every few days or so until its at a point where you like it. The more batches you use it for, the less will remain so it will probably take longer, but I think the only way to know is to test it out.
The other option, if you're so inclined, is to turn it into a sour barrel. Not sure what the whisky/oak would do to sour beers so you may want to either purge with water a little or do a few regular batches before souring it up. At that point, toss a good base beer into the barrel, add some bugs (brett, pedio, maybe some lacto, etc.) or some dregs, and let it sit for a long while. There are a ton of posts on here from people that know way more about sour bugs than I that could help you out, but if you're interested a 10-gal barrel is probably the best way to jump in feet first!
