If you wrap your hair at night to protect it and don't use too many products that may cause build up while it is straight it may last longer. Also I usually avoid working out while my hair is straight and sweating so that my hair doesnt curl. I also use cooler water when I shower which I hate but the steam from the hot water would wreck my staright hair.
I flat iron my hair 2-3 times per year and when I do I can keep it straight for about a week to a week and a half. All I do is wrap my hair at night and in the morning comb or finger comb. I don't apply any more products unless it is a light serum for the ends.