What Triggers a Rebalance?

What triggers a rebalance? How will I know when it does make sense for me to get back to target? Is it going to be a time period thing. Like every six months, I'm going to place a trade. Am I going to add some tolerance bands saying, okay, I want to be 50% allocated here, but if it goes to 45, I's going to buy more. Or is it just going to be something that I have it set it and forget it automatically? We'll talk about this in a moment.