Last summer I aged about 2 pounds of beans on oak, and if memory serves, this was the process I settled on: .25 oz of oak cubes/.5 - 1 pound of greens; store in sealed mason jars 3-7 days as desired. When I went longer than a week, the result was a tannin bomb, which really surprised me given the small amount of oak I was using. As Tom mentioned, the beans will pick up all flavors (wood, spirits, wine) quite quickly, so it might be wise to begin on the low end to get a feel for what you prefer.
I just used oak cubes from the homebrew store, and I experimented with American, French, and Hungarian varieties, all medium toast. The distinctions brewers seek among the oaks (e.g., more or less vanillin, etc.) seemed largely lost, and if I were to do it again, I'd likely experiment with different woods like Tom has, and I would second the recommendation for clean, less expensive coffees to be used.
Think the most enjoyable concoction I produced was a really clean washed Ethiopian (one that I wasn't really dazzled by) aged on American oak cubes that had been soaked in cognac. The same coffee aged on Pinot-soaked French oak was also quite pleasant. These were both especially nice iced.
A friend of mine runs the barrel program at a brewery in San Diego, and they age beans (typically Harrar and a Java) in 5 gallon spirit barrels for their coffee stout. While I enjoy playing with small amounts at home, I give him a hard time about aging beans like this as being an abomination--jokingly of course--and I sometimes think this when I see the things some roasters are chucking in barrels. I've had a few of the barrel-aged offerings from Ceremony, and while they are quite interesting, I've never had anything I'd order a second cup of.
All that being said, it's still a lot of fun to play around with if done responsibly