About 2 or 3 days in total. I had to pre-buy a hub and crack it open. I used this one:
because it came with 4 arms that could be bent to fit. I removed the core from the cube and cut off the back of the centres, just leaving a square and the cap. I had to glue the cube together and hollow out portions of it to fit the circuit board and I managed to hot-glue that into place without harming the connections.
I needed to re-solder the USB male connection because the socket wouldn't thread through the centre. I then assembled it all. I originally intended to have the sockets end at the centres and have them recessed inside, but the wires meant they were too far out. So I had to replace the hands / feet of the robot.
Then I copied the sticker pattern off a scrambled cube and cut the centre stickers to size.
There was a little bit more fiddling than all that, but that covers most of it.