The simple answer is....when you don't see any further improvement.....there are as many ways to finish acrylics/ plastics as their are species of wood. Every one has their own views on the right approach....wet sand, dry sand, buffing wheel....as above what you are trying to do at every step is to remove the scratches from the previous grit.....what ever and how ever you achieve that, that's the goal....
Good practice is to start with the finest you can use to remove tool marks, with timber and a good skew you may not even need to sand, but this is acrylic....when you see no more improvement in the finish it's time to change grits.....but first wipe the blank to remove any trace of the current grit, then change. I always sand radially and laterally before I wipe the blank, I also use liquid polish when I get to the final stages as I have previously said we all use different methods.
Good luck......also some colours can be worse than others for showing scratches......black for one is a nightmare
