My hair has been straight for approximately 5 weeks now. Every time I wash it, I anxiously check to make sure there are no heat damaged pieces. You will know your hair is heat damaged because it either won't curl back up at all, or it will have a looser curl pattern than before. Using a heat protectant and as little heat as possible is so helpful. Everyone is different - because I have friends who straighten their hair often but don't get heat damage, and then there is me. My hair tends to suffer from this easily (which is why you haven't seen me straighten my hair more than 4 times in the 4 times I've been natural).
I've been using and happen to really like Moroccan Oil and Organic Root Stimulator's Heat Protectant. I apply the ORS heat protectant prior to blow drying and the Moroccan Oil prior to using the flat iron. I don't pass the flat iron more than 2 times on each section.