It depends on the size of container/pot and tree species how often a Bonsai tree needs to be repotted. Fast growing trees need to be repotted every two years (sometimes even every year), while older, more mature trees need to be repotted every three to five years. Do not repot on a routine, instead check on your trees every early spring by carefully removing the tree from its pot. A Bonsai needs to be repotted when the roots circle around the root system. When the roots are still contained in soil wait another year before checking again.
Repotting work normally needs to be done during the early spring; when the tree is still in dormancy. This way the somewhat damaging effect of repotting on a tree is reduced to a minimum, as the tree does not yet have to sustain a full-grown foliage. Repotting in early spring will also ensure that damage done to the root system will be repaired soon, as soon as the tree starts growing.
Choosing the right soil mixture is crucial for the health of your trees, it should be draining enough to prevent the roots from rotting, while absorbing enough water to supply the tree with water.