I made an oak-aged barleywine. It was English style with a lot of crystal malts. I used 0.45 oz of French Oak cubes per gallon and left a lot of the char on. I let that sit for three months. The results were exquisite. I have tasting notes from a bottle that was about 11 months old and one that was about 13 months old. By 13 months a lot of the oaky flavors became vanilla and tobacco-like. This is definitely a beer I will be enjoying years from now.
In short, use cubes, age for a long time, you will not be disappointed.