Repotting seems to be a daunting operation to many beginners of the bonsai world. There is no set time frame to repot your bonsai as it depends on age and the trees healthy vigor, but between 1 - 5 years is a good guide on average.
Upon simple inspection of the tree, does it retain water on the surface? If this is the case it may be pot bound, meaning the roots have totally filled the pot. The tree needs the correct treatment, trimming of roots and replacement of the appropriate soil mix (depending on species). This varies and also needs to be done in the correct season.
In bonsai, just cutting the root mass alone does not mean small leaves and a small tree. We are seeking to minature the whole tree, which means looking to achieve fine ramification of the branch structure and through correct root reduction. This can usually be achieved over several growing seasons.