As a hairstylist, I get asked this question a lot and it's not a one size fits all answer. To look into this we have to determine if you have long hair, short hair, or maybe somewhere in-between. Maybe you are growing to reach an epic hair goal or maybe you love to have fun with hair color or blonding. These are just some examples of what might determine when you should get a haircut.
The two biggest questions I ask are. Is your hair healthy? And by that I mean. Is your hair dry? Does your hair have dead ends? Does your hair have split ends? If you answered yes to any of these you may be ready for a haircut and maybe even want a conditioning treatment too(keep an eye out for conditioning treatment post). The second question is a little easier to identify. Does your hair look good? This could be in a couple of different contexts. What I mean by that is. Is your hair overgrown? For a short haircut, this could mean you have unwanted growth over the ears or the nape of your neck. If you have a longer style your ends may start to look a little stragley or your layers could be starting to get too heavy or you may see weight starting to build up where you don't want it.
At the end of the day, there are a couple of windows I recommend to my clients depending on each of their personal situations. If you have a fade or pixie haircut I suggest 3-5 weeks depending on how clean-cut you want to see your style and how fast your hair grows. If you have medium-length hair I like to see client between 4-6 weeks. This keeps their hair at the medium length but prevents the hair from getting any unwanted weight lines or flips and kicks when the hair becomes shoulder-length hair but is barely hitting the shoulders. If you have long hair we have to divide people into two groups. If you have natural hair and let your hair air dry or use just professional tools on your hair lightly you can get away with the biggest gap depending on your personal preferences. For this group, I'll recommend 8-15 weeks. This allows us to keep the perimeter clean while preventing dead ends, split ends, and from the overall look starting to get stragley. Now if you have long hair and do chemical services, use professional hot tools on the regular or less than awesome hot tools in general (non-professional tools can cause more damage than professional tools. To be discussed in another post later on) then I strongly encourage a more frequent hair cut because we are going to see dead ends sooner than all the other styles. For this group, I like to see them 6-9 weeks depending on what their hair is going through in-between haircuts. That way we can keep that gorgeous long-hair looking luscious and free of dead ends, split ends, and stragley or stringy perimeter.