At what point do plants stop growing in a pot? It's a bit involved, but the simple answer is that new growth stops or slows when they become rootbound. So what's rootbound? Rootbound is when there is no effective space for new roots to occupy. Roots effectively occupy the entire volume of space between the soil particles. One of the first symptoms of being rootbound is, in fact, that plant growth slows despite favorable environmental conditions (light, water, fertilizer, etc). The second symptom is that rootbound plants begin having difficulty taking up fertilizer. This is undoubtedly related to the inability to form new root tissue. You see this as a chlorosis despite the fact that they have been properly fertilized.
People often confuse leaves with new growth. New growth is the process of continually opening the terminal bud of a stem (shoot formation). Plants can be, and frequently are, alive and relatively healthy with absolutely no new growth. This happens when plants are severely rootbound, there is a lack of fertilizer, or after a trauma such as barerooting. The existing buds will open, leaves will form, but no shoots will develop. This condition will persist until conditions change. I have seen many plants survive year after year without shoot growth. New growth each year consists of a succession of opening terminal and axillary buds in the spring without any shoots to form an internode. If you look closely at the stems there are just a pile of leaf bundle scars piled up on one another. Talk about close internodes!
Rootbound plants need to be rootpruned and shifted at the nearest appropriate opportunity. This usually means winter, because unlike shifting an intact rootball, rootbound plants must be rootpruned to initiate proper root growth.
It is difficult to tell when a plant is rootbound just by observing the roots. I think it is better to determine 'rootbound' by both the symptoms of growth (or lack thereof) and the physical density of the roots. For our purposes (bonsai), trees should be rootpruned and repotted long before they reach rootbound conditions. This doesn't happen overnight. There is a long gradual procession of slowing growth over time, usually several years before all new growth stops. It is clearly evident what is happening if you stop to look.