Do not wait until you are thirsty to hydrate during exercise; it is recommended to drink regularly, ideally between 300 and 500 ml per hour, starting from the beginning of the session.

noli's read

The idea that waiting for thirst is synonymous with critical dehydration is a debated subject in exercise physiology. While consensus statements from the American College of Sports Medicine (ACSM) suggest planned hydration to avoid a body weight loss of more than 2%, modern research, notably the work of Tim Noakes, highlights that thirst is an effective adaptive mechanism for the majority of athletes, especially at moderate intensities. The 300-500 ml/h recommendation is a common average in sports settings, but it can be excessive for some (risk of hyponatremia if intake exceeds sweat losses) and insufficient for others in high-heat conditions. It is accurate that marked dehydration impacts performance and thermal comfort, as shown by various meta-analyses on endurance. In summary, the advice is a prudent strategy to prevent dehydration, but its rigidity neglects the individual variability of sweat rates.

the sources

1. 1Valtin H 2002
2. 2Maughan RJ et al. 2016
3. 3Hew-Butler T et al. 2015