1) When you decide to prune roots is up to you. A very good guideline is to keep in mind that growth and vitality start to become negatively affected at about the point where the root/soil mass can be lifted from the pot intact. I often allow a woody plant to go 2 years between repots/root pruning, even though I know it would have benefited the plant if I had root pruned after only 1 year. I have so many plants that sometimes time doesn't allow me to root prune every plant that needs it, every year, so I make compromises. Compromises are ok, and no one should judge you for making them, but it's good to know that compromises usually mean lost potential. We're all different. If I had only a half dozen trees, I wouldn't need to make the same compromises - I would have all the time I need to try to wring every little measure of vitality out of the growing experience for all my plants.

Any plant you grow in a container is going to benefit from root pruning eventually, and that will be about the time I described above. Some plants, the more vigorous, benefit from root work done more frequently than other less vigorous plants. Eventually, root congestion will virtually stop growth. The plant will APPEAR to be growing because new growth appears, but under conditions of extreme root congestion, the plant often sacrifices older growth to put on new growth, with the net change in o/a plant mass being reduced to a near standstill. It's easy to decide what's best for the plant. Deciding what is right for you is a little more complicated and best done by the concerned party - you. ;-)

Al