I find regular trimmings very "healthy" for my hair. Tried to go w/o them and ended up having to chop off 2 inches to get my hair to look thick throughout its length. So 6-8 weeks seems to work best for me. The secret is to keep replacing them if they get blunt, and not to use them for anything else but your hair.
See, everybody's hair is different, it is good to learn what works for you and stick to it... take care
