I first boiled 1.15 oz. French medium toast oak chips in 1/4 cup water for 9 minutes, and added just the chips to secondary (full 6-gallon carboy) of Denny's Bourbon Vanilla Imperial Porter for 10 days before bottling. It was heavy on the oak until it integrated well several months after bottling, and then was really delicious although the vanilla beans faded by then - that beer got the most compliments of any this year, and the most requests to brew it again. I would rather err on the side of too little than too much oak, and a little goes a long way.
Of course cubes have less surface area than chips and so can stay in longer but I would still steam or boil them for awhile to take the edge off a bit, and maybe taste it every week after the first one to two weeks (YMMV depending on how much you start with).