Pruning your bonsai is necessary to keep it in a compact shape. The more you prune, the more branches your Bonsai will produce. Various plants develop at varying rates, the frequency with which you must prune them will vary depending on the sort of plant you have.
In general, for most plants, you should allow the new growth to extend to 3 to 4 sets of leaves before cutting it back to 1 or 2 sets of leaves with a sharp pair of bonsai trimming scissors.