I did the whole modification within an afternoon (< 4hours) even allowing for the problem I had.
Just make sure you are fully familiar with the modification’s kit instructions (I had downloaded them before I brought the kit). Apart from the grub screw problem I had, the trickiest bit would have been removing the wiring loom to extract the motors, not difficult, mine were wrapped around other wires, a pair of tweezers were useful when putting the loom back. Without the grub screw problem it would have take just over an hour, and I was taking my time and double checking everything.