The American College of Sports Medicine recommends that endurance athletes consume 300-600mg sodium/hour (1.7-2.9g salt) during prolonged exercise 
. However, sodium losses vary from person to person depending on factors such as exercise intensity and duration, ambient temperature, body size, age, sex, and clothing, and sodium intake should be individualized to the athlete.
 Vitale, K., & Getzin, A. (2019). Nutrition and Supplement Update for the Endurance Athlete: Review and Recommendations. Nutrients, 11(6), 1289. https://doi.org/10.3390/nu11061289
 Nikolaidis, P., Veniamakis, E., Rosemann, T., & Knechtle, B. (2018). Nutrition in Ultra-Endurance: State of the Art. Nutrients, 10(12), 1995. https://doi.org/10.3390/nu10121995