Posted: 27/12/2013 at 21:08
Firstly you can cut as hard as you like. It will regrow very happily. Mahonias do naturally become leggy, even ugly. I have rejuvenated quite a few simply by cutting hard back. One was almost treelike in a friend's garden....thin, leggy and awful. It is now bushy and free flowering. Prune every year to keep compact.
Its hard to say how many feet it will grow back in one year. How tall is it now? I would expect a 10' shrub cut back to 5' would grow at least 2'. I cut a mahonia that formed part of an open boundary to 2' using a sharp saw. It is now kept to 5 or 6 feet.
Prune after flowering.....actually I would delay until early spring if it's flowering now.