However, I can tell you that people's hair can get greasy if they don't wash it for a day. This is because the scalp produces natural oils (sebum) to keep hair moisturized. If you don't wash your hair regularly, the oil can build up and make your hair look and feel greasy.
How often you should wash your hair depends on your hair type and lifestyle. Some people can go a few days without washing, while others need to wash it daily. If you're not sure how often to wash your hair, it's best to talk to a dermatologist or hairstylist.