It only took me about 30 minutes at most to straighten the hair of my clients who had normal curly hair. There are many factors to straightening hair that I can't possibly give you the answer you need without knowing what texture or condition your hair is in before you begin the straightening process.
I have not seen your hair or felt its texture, so I can only tell you to make sure you use the iron without rushing or using a hot iron on large sections at a time because you may not have a lot of time to straighten it. Take your time and allow enough time to do a thorough job. Always work with very thin sections of hair, the thinner the sections, the straighter you will be able to get your hair.
The thing about naturally curly, wavy hair is, it reverts to its natural state easily, so don't expect your hair to remain straight for days or weeks on end. Humidity will make your hair revert to its naturally curly wavy state in a few minutes if the weather conditions are right for it. I hope this reply has been helpful to you.