You want to repot before the mix decomposes, that will depend on the type of mix it is. 2 years is a general rule of thumb but sphagnum moss i would repot every year, a coarse Ochiata back can go a few years.

The best time to repot is when you see new roots beginning to form, which should be early spring. If the plant is over due for a repot you can do it anytime to save the plant.

Thank you, James.