How long does the effect of hair treatment such as ...

The only pitfall of the keratin treatment is the cost. Keratin treatments don't have a set length of time that they last. It depends on a few factors, your natural hair texture, how often you wash your hair, and how much of the product is applied to your hair. Here is an example, I have naturally straight hair, so a stylist is going to place a minimal amount of the product on my hair for a short length of time, because the only reason for me to get a treatment like this would be to seal damage to cut frizz and create more shine. I wash my hair everday, because I'm pretty active. The treatment will last me a shorter length of time, because of the application and my hygiene routine.

Most interesting part is that BKT claim smooth silky hair using Keratin, but it seems keratin has nothing to do with hair straightening and does NOT change the structure of the hair and it's really the formaldehyde that makes the treatment work. According to Allure Magazine "It's doubtful that keratin does anything except provide a good marketing story. It's window dressing."