It depends on the effect of dyeing your hair. If your white hair is dyed black, you can only dye your new hair every month. But if you dye it a trendy color, you need to bleach it multiple times and then color it. After a few times, your hair will be very dry, so you need to think carefully. Everyone's needs are different. Some people can't help but want to re-dye their hair as soon as their black hair grows out. It doesn't matter if some people have obvious continuous or triple colors on their heads. It all depends on personal needs.
Whether it hurts the body or not is an unclear topic. My math teacher is 75 this year. He started dyeing his hair when he was in his 20s. He dyed it once a month. Now he has dyed it for more than 50 years. He dances twice a day in the morning and evening. He travels once a year. He is in good health. Get older and get stronger.
People who love beauty and pay attention to image will dye their hair more frequently. It is normal to dye their hair three or four times a year. As long as your hair can handle it, you can dye it as many times as you like. However, the hair dye products you use must be qualified. Do not use fake or low-quality hair dyes from other brands. Those may contain metallic pigments and may cause harm to the body if used for a long time. Regular products are expensive, but all organic pigments are harmless to the human body and can be used with confidence. Apart from damaging the hair and causing allergies in a very few people with sensitive bodies, there is nothing wrong with hair dyeing.
As for the reports of hair dyeing harming the body, none of them are caused by the use of regular brand products. They are all caused by the use of unqualified, unprofessional and inferior hair dyes. If you need to dye your hair, it is best to use a well-known brand. Although the price is dozens of times higher than fake products, it is very safe.
The maximum number of times you can dye your hair a year depends on the products you use, the effect you want to achieve, and the method you use to dye your hair. There is no unified rule and it may vary from person to person.