You likely already know that daily heat styling isn't recommended. But when it comes to keeping your natural hair as healthy as possible, keep in mind that everyone's hair isn't the same. Whether your straightening routine works specifically for you is more important than any blogger or YouTube guru's advice. However, if you know your curl pattern, hair type, and how damaged your hair is, you're at a good starting point for knowing how often to straighten your natural hair.
How often you can safely flat iron natural hair depends a great deal on the condition that your hair is in. If your mane is in any way dry, under-conditioned, damaged or in any other less-than-healthy state, flat ironing will likely make things worse. A good rule of thumb is to consider what your hair has been through—if it's been colored, or chemically straightened recently, it's probably more than a little damaged. Therefore, It's not recommended you apply any direct heat to your hair. If, on the other hand, you're good about keeping your hair protected, you can work out a flat iron schedule for you.
It's generally suggested that heat styling be done no more than once per week. Natural hair should always be freshly shampooed, conditioned and completely dry before thermal styling. Straightening dirty hair with a flat iron will only "cook" oil and dirt in, which will lead to more damage. Even on a once-per-week regimen, heat styling is still never actually good for your hair, so you'll need to consistently keep track of your hair's health. It's the best way to ensure you're not getting a lot of split ends, and that your curls don't become excessively dry or brittle.
If you haven't been using a flat iron with adjustable temperature controls, get your hands on one before the next time you intend to straighten your hair. Without being able to control how hot your iron is, you won't be able to adjust heat according to your hair's specific needs. Using too-high heat, even just once a week, will still lead to dryness and damage. If you hear "sizzling" or smell burning when you touch an iron to your natural hair, even once, it's way too hot. Also, invest in a heat protectant known to be good for curls, like Briogeo's Farewell Frizz Créme ($24).
Of course, life doesn't tend to run like clockwork, so you probably won't have an exact weekly straightening schedule. In order to minimize heat damage as much as possible, give your tresses periodic rests from any thermal styling; going a few weeks without heat can do a lot for your hair. Look into low-manipulation protective styles that allow your hair to fully recover from the effects of heat. You might find flat ironing once monthly is better for your hair—in general, the less direct heat you apply, the better for the health of your hair.
However much you heat style, regular deep conditioning is a must to prevent dryness, and you should be using protein treatments like Aphogee's ($24) or DevaCurl's Melt Into Moisture Mask ($36) to strengthen your locks. Learning how to balance the moisture and protein levels in your hair will help you keep it strong and hydrated; healthy hair is much less likely to suffer damage and breakage from whatever you do to it, including heat styling.