Back when I lived on the West Coast I had a brief foray into bonsai. The climate of everywhere else in Canada is not conducive to bonsai but I did learn a few things about growing trees in pots. I've been growing this lemon tree for around 15 years, from seed, it is now as tall as me and wider than the average doorway. Of course this is an indoor plant in the winter as growing trees and shrubs in our zone in a pot is an entirely different challenge and article!
Like all pot grown plants there comes a time when the roots fill the entire space, pot bound in other terms. When the weather is warm enough and the plant begins growing actively follow these easy steps about every 5 years or so.
Trim any wayward or lanky branches back into a more compact shape. Water well and remove root ball onto a tarp or some such dirt catching surface.
Take a saw or large serrated knife and remove 2 to 3 inches of the thick root ball from all sides.
Place plant back in the pot, fill in the sides with fresh soil. Water well to make sure there are no air pockets. Also a great time to fertilize with a well balanced fertilizer or whatever your plant requires. You do not need the supervision of a Border Collie but it can't hurt!
Do this every 5 years or so, whenever you think your tree might be too pot bound and you can grow indoor/outdoor trees for many years.