Hi W0od,
In my basic system, I rebalance my positions only once a year, at the end of December - as long as they have changed at least ± 10%.
My testing has indicated that more frequent rebalancing is not worth it, unless you want to try to optimize it to the market cycle.
Stocks do enter volatile periods where monthly rebalancing can pay off. They also enter multiyear periods (such as the 1982-2000 bull market, where it's optimal to only rebalance after several years.
I personally want to avoid optimizing, and want to be able to trade many stocks, so I want to stick with only one rebalancing frequency. If you want to stick with one, I think yearly is the most robust - it works in most situations.
But, in my book, in the section on variations, I mention that you can change the rebalancing frequency, or use a rebalancing percentage trigger instead (such as rebalancing whenever a stock moves 15% or some other percentage), or combine rebalancing frequencies and triggers (i.e. rebalance yearly if it has moved at least 10% from the last rebalance, as well as rebalancing if it moves 15% or some other percentage in between).
The main thing is to build a personal system that you feel comfortable enough and trust enough to use, without letting fear and greed affect your trading.
No matter what frequency you pick, the hardest part is to resist breaking your rule and rebalancing right after big drops or rises.