A. The frequency of balancing a portfolio depends on your investment stage.
If your portfolio is still in the accumulation stage, try to avoid rebalancing frequently (e.g. do not rebalance every half year or even once a year), because at this stage you should be able to sustain a large deviation from your original stock/bond ratio by keeping the winners for as long as possible, and making up any loss with new fund contributions.
If you are already in the distribution stage, portfolio rebalancing is very important, because any loss won't be made up with new funds, you want to cut your loss, especially stock loss, short. In this stage, you might want to consider balancing twice a year.