How do you decide how often to rebalance a long-term portfolio?

Many beginners set up an allocation once and then wonder whether they should ever touch it again. Rebalancing is one way to keep risk where you intended it, since a stock-heavy mix can drift well past its target after a strong run. But rebalancing too often can add trading costs and taxes, and it can turn a long-term plan into a series of small decisions. Some people rebalance on a calendar, such as once a year, while others wait until an asset class moves a set distance from its target. Each approach has trade-offs in simplicity, cost and how much attention it demands. I'm not sure there is one answer that suits everyone, and the right choice probably depends on account type and how much time someone wants to spend on it. What rule, if any, do you use to decide when it's time to rebalance, and how did you arrive at it?