What's a fortnight?
I think it depends on your hair. Some can take it, some can't. I use to flat iron about once or twice a week. I retained no length because my hair was always being manipulated (and this is with air-drying). I also had split ends and thin hair. When I combed my hair, broken pieces would be all over my floor and counters. I flat iron about once a month or once every two months now; I usually wash n go. No split ends, very little breakage, and my hair is thicker
Once a week, if it is only the newgrowth "area' and not the entire length!