The strength and condition of the hair is the most important factor when deciding how often you should color your hair. Some people's hair can withstand several chemical processes in a short period of time, while other hair types are much more fragile. Those with thick and coarse hair types can get away with frequent coloring than those with fine hair, since it's much easier to damage hair that is fine or thin. Porosity of the hair is another consideration, as it will affect processing time. Porosity is the hair's ability to absorb liquids. Just like a sponge, hair that is overly-porous will absorb dyes faster than normal hair. In order to test your hair's porosity, snip a few strands of hair near the ends and place them into a cup of water. Within a few seconds, porous hair will sink to the bottom, while healthy hair will float.
The type of hair dye being used is another important factor. Most permanent dyes contain ammonia, which can be damaging to the hair. Temporary and semi-permanent dyes are much less damaging, but they only have the ability to darken the hair, not lighten it. Lightening the hair is always more damaging than darkening the hair, because this involves stripping pigment out of the hair rather than putting pigment into the hair. Provided that your hair is healthy, strong, and has passed the porosity test, hair can be safely colored with a permanent dye every four weeks. A semi-permanent or demi-permanent color (sometimes referred to as a color glaze) can be applied every two weeks if desired.
Hair that is over-porous or dry can still be colored, but less frequently and with more care. Close attention must be paid to processing time, since porous hair will process faster than healthy hair. Unless the hair is extremely damaged, dry and porous hair can be colored every 6-8 weeks with a permanent dye and every 4-6 weeks with a semi-permanent dye.
Severely damaged hair should never be colored at home. Only a trained professional will be able to properly assess the condition of the hair and, after a thorough analysis, he or she will be able to decide whether or not it is a good idea to apply color to the hair. Signs of severe damage include breakage and excessive hair loss. Applying the wrong type of hair dye or leaving the dye on the hair for the wrong amount of time can result in a hair disaster, which is why it is always best to leave coloring in the hands of a professional.