The real question is how long you could be on a bus. This is already a 16 hour trip (as opposed to a 2 hour flight). Most bus companies try to stop along the way to pick up and drop off passengers so this would be an extremely difficult trip to make direct. At least if you take the bus, you will be able to see the country in a way that many have not and will not. Once arriving in Cape Town, what did you do?