How often should I color my hair?

Varies. General rule for bleach based hair color agents is not to bleach more often than every two months. Extenuating circumstances such as home versus salon bleachings, caliber of your hair and other hair care practices need to be factored in to this generally accepted interval.
Do not overdo. Colr your hair when the roots show. Bit do not colr too often and although uncommon make sire you are not allergic to the hair dye. You can become allergic even after using multiple times. You are supposed to patch test on your earlobe or neck one day before reapplying color.