The keratin sticks to everyone very differently. The manufacturer says is will last up to 4 months. This should mean that in 4 months, the keratin is gone. However, that is certainly not true for everyone. Keratin can stick permanently to some hair, while it can completely dissolve off of others within a couple months. IF you are a person who's hair loves keratin, you may not have much luck removing it. But, if you're really motivated, you could do the following:
1.switch all your products to ones containing salt/sodium chloride.
2.dissolve salt into water, in a spray bottle, and spritz the hair down and let it sit as long as you can. Repeat daily or whenever you like.
If your problem is more about blending the curly roots with the straight ends, and your ultimate goal is having the healthiest hair possible, try going wavy with the ends, not flatironed at the roots. Hot rollers do a nice job of adding wave. Whereas flatironing the roots only starts the bond breaking process that makes curly hair more fuzzy and un-uniform in pattern.
If your goal is to still have some wave in the hair, but the ends are too straight due to keratin accumulation, you can use products with salt in them, only on the ends, in order to pull out some keratin. This helps even out the wave overall.