Estimate, based on his ratings progression, when he will be at a point where he is ML ready, or when you will want/need him at the ML level. Then step him through the minor league levels at an appropriate pace.
For instance, if he's in Low A and you feel that you won't want/need him in the majors for another three seasons, let him spend one season at each intermediate level (High A, AA, and AAA). If he's in RL and you feel that he should be ML ready in two seasons, then let him spend approximately 1/2 a season at each of the four minor league levels between RL and ML.
It's not a good practice to base minor league promotions (or lack of) based on minor league success/failure. By doing that, your decisions are being influenced by how well/poorly other owners are running their minor leagues.