Yes your curl will come back after a keratin hair treatment. How long it takes depends on whether you have a full keratin treatment or the express, and how often you wash your hair. Other things like swimming and working out can also affect how long the treatment lasts.
Keratin is added to your hair and over time it dissipates. This is different from chemical straighteners and relaxers that change the bonds of your hair permanently. One of the things that takes keratin away quickly is salt, so things like sweating from a workout or swimming will make your curls return faster as it strips the keratin away.
Typically curls will return within 3-6 months with a full keratin treatment, and 3-6 weeks with an express treatment. The more you have treatments done, the longer they last. For me, I do the express and my curls are barely returning after 6 weeks, so I could probably go longer in between treatments, but the new growth of course is curly so I like to stick with every 6 weeks.