This depends on the hair building fiber brand. For example, Toppik claims that hair fibers last for a full 24 hours. Others may only last for a couple of hours, depending on the activity. The real issue is that hair fibers are susceptible to moisture and may start to wear off if sweat, rain, or other moisture gets in your hair. For long-lasting use, it’s best to wear hair fibers when you’re headed indoors with air conditioning. Even waterproof or water-resistant hair fibers have issues with washing away and revealing bald spots if exposed to moisture for long periods of time.