Bear in mind this is for oak cubes, but I've heard a good starting point is between one to two ounces for at least two to three weeks. The lower the amount, the less oak flavor you'll get in a longer amount of time. The more oak you add, the more oak flavor you'll get in a less amount of time. Cubes have more surface area (therefor less contact with the beer and less/slower extraction) than chips, but less than spires or barrels, so your mileage may vary depending on what you read or hear from people. Also understand that people's responses to this will vary depending on how much oak they personally like, which may not match what you like.
With that said, your best bet is to start probably at 1-1.5 oz, let it sit for two weeks, draw a sample, and see if it is to your liking. If not, let it sit longer (or even add more oak). Don't be surprised if it takes in the upwards of months to get the flavor you're looking for, as the oak flavor will change over time. Be careful to not add too much oak, as it may impart undesirable intensity. Luckily if you do happen to over-oak your beer, it sounds like you'll have an oak-less batch that you can blend back in to the oaked batch(es) to help subdue the error.