This can differ between our different portfolios and their level of aggressiveness in the market. We rebalance when the market has moved your portfolio away from its target allocation by more than a few percent. Also, we review and update our models regularly to balance between our long-term investing approach and changing market conditions.
As you add cash to a portfolio, we buy ETFs to maintain your target allocation. This also will happen when dividends are issued. The funds that are added in will be spread out amongst the proper allocations to keep you balanced properly. To keep transaction costs low, we won't buy a small amount of a security if it results in unnecessary transaction fees.
If you want some more details on how invest, check our our methodology. Section 4 pertains to rebalancing if you are looking for a bit more.