Often enough to maintain healthy ends and keep your hairstyles manageable. Hair grows an average of 1/2" to 3/4" per month, but not each hair grows at the same rate-periodic trims will prevent hair from looking ragged (usually every 4-6 weeks for short styles and 6-8 weeks for medium to long styles). Chemically-treated hair is particularly vulnerable to harsh environments and may need to be trimmed more often.

If hair is not trimmed and starts to split, it will split up the hair shaft and cause breakage. See Hair Health