Hiya,
Don't know if this will help but at the salon I work in, we recommend the client comes in every 2/3 weeks for a rebalance (infill) and after that in another 2/3 weeks for a complete rebalance (which includes backfills).
Usually after two weeks they don't need backfills but again it varies for each client - some client will have quick growth, especially after having a set of nails put on.