With room for roots to run, the plant will or should show a high level of vitality (all other cultural conditions being favorable) until the point in time when root congestion gets to the point where the soil/root mass could be lifted from the pot intact. From that point forward, root congestion will limit growth, vitality, and the plant's ability to defend itself and a gradual decline will start and increase in direct proportion to the level of root congestion. The reason this occurs is, with a pot full of roots and the organic portion of the medium gassing off as CO2 as the medium is broken down, there will be little room left in the pot to hold the essential resources of air (oxygen), water, and nutrients.
Plants have a pecking order insofar as the strength of their energy sinks. Sinks are parts of the tree that get the largest share of the plant's food/energy. Energy is first allocated to respiratory function, i.e. to maintenance of living tissues, then, to production of fine roots, followed by flower and seed/fruit production, then primary growth (extension of both roots and shoots), then secondary growth (thickening), and finally, the synthesis of defensive chemicals. Chemical defense is last in the pecking order because synthesis of these chemicals is a byproduct of the plants metabolic rate, which is why weakened plants usually succumb to disease pathogens or insect herbivory because they have little in the way of natural defense.
Being realistic, I think you should be willing to repot the tree every 3 years at a minimum if you want to have it for the long term; or, be prepared to enjoy it temporarily until its decline is conspicuous enough you'd want to replace it. You could pot up to a larger pot, but that is a half measure which leaves the lion's share of the soil/root mass still horribly congested.
This is a maple 4 years in the same pot. I didn't get it repotted this spring because I've been repotting bonsai while trying to get a new boat set up with radios and all the electronic gadgetry one needs (or at least wants) while on the water. I'm seriously concerned about how steep this years decline is going to be. I know I have to make it a first priority next spring if I want it to continue to survive - thrive is already out of the question. This is summer last year and it's already showing signs of serious decline.
We had a stretch of really warm weather in early Feb that really screwed things up for bonsai trees - getting them growing that early it tough when there is no light and its too cold after the warm spell to move them in and out.
Al
Q