Keratin treatments vary in longevity. Depending on the product being used, keratin can last anywhere from 6 weeks to 6 months.
A keratin service that lasts on the shorter end (6 weeks) tends to be a product that could be stronger. Using this product will not get hair completely straight like other keratin services. This keratin treatment is meant to reduce frizz rather than straighten hair. If you are looking to straighten hair, you will want to go with keratin that lasts longer (6 months). Ask your stylist which product they use and what the longevity is for that product.
Something to keep in mind if you get a keratin treatment, no matter which one, washing your hair less will increase the longevity of your results. If you wash your hair only a couple of times a week, you will get longer out of your treatment. If you are someone that washes your hair frequently, your keratin will only last for a while. Also, you should not wet, wash, or tie your hair back right after your treatment. The longer you wait to do these things, the longer your keratin will last. Depending on the product, you should wait at least 48-60 hours to wash or wet hair after treatment. This is something you should discuss with your stylist.
A sulfate-free shampoo and conditioner is also key to keeping your keratin longer. Sulfites will break down the keratin in your hair which will cause your hair to revert to curly and frizzy. Avoid chlorine swimming pools and hot tubs or anything with chlorinated water. Chlorine is another chemical that breaks down keratin.