Keratin can be used as often as you like. Keratin is found in your hair naturally, so this treatment enhances what you already have. I suggest if you have more questions to google Brazilian hair treatments, ( as it is very similar with the exception that it uses formaldehyde), you might find your answers there.
Iwash my hair daily, so I may have to do the treatment sooner than 4 weeks.
This is a great product and very, very happy with the results. It is as good as the Brazilian!