They need to be repotted at about the time the root/soil mass can be lifted from the pot intact. There is a distinct difference between repotting and simply potting up, with repotting being the choice that most benefits the plant, offering the only opportunity between the two that the plant might grow to it's genetic potential. How often the plant reaches the point where it needs repotting depends on your skills as a grower and the volume of soil it was planted in. The best soil is one that is durable (retains its structure), drains freely, and supports little perched water at container capacity (when the soil is fully saturated). For a better understanding of how the amount of water your soil holds affects your plant's growth and vitality, follow the link I left below.

Al

Here is a link that might be useful: More about soils if you click me!