A once-a-month guideline is a good suggestion, but it really depends on what you put on your hair from day to day. People who use a lot of heavy, petroleum-based products in their hair need to clean their combs and brushes more frequently than those who stick to more water-based products. If you use your hairbrush daily, pull any hairs out of the brush when you're finished styling your hair. That way, when it comes time to wash it, you'll have less hair to pull out at that time. The same goes for your comb: after grooming, get rid of any hairs that are caught in it.
The more natural products you use, the less likely you'll experience that gunky buildup that's sometimes impossible to completely remove. If comb and brush cleaning is one of your less favorite tasks, then consider your product selection. As long as you don't have heavy, greasy buildup in your hair care tools, cleaning them about once a month or so is fine. If, however, your combs and brushes are often laden down with heavy product residue, a more frequent cleaning schedule is in order; this may mean once a week or biweekly.
In some cases, it's nearly impossible to remove the buildup in a brush's bristles. This is usually the result of very infrequent cleaning along with the use of heavy, greasy products. In this case, it's best to discard the brush and buy a new one. Don't forget to clean the new one more frequently! If you don't know how to do it, this tutorial walks you through this simple task.