The three phases of event hydration
Hydration around competition is usually planned in three phases. Pre-loading aims to start the event well hydrated, with guidelines often suggesting roughly 5 to 7 mL of fluid per kg of bodyweight a few hours beforehand. During the event, intake is matched to sweat rate, commonly in a band around 0.4 to 0.8 L per hour. Afterward, replacing more fluid than was lost, often near 1.5 L per kg of body mass lost, accounts for ongoing urine losses.
These numbers are starting points. The most reliable individual guide is measured sweat rate, found from body-mass change across a session adjusted for fluid taken in.
Why sodium and sweat rate personalize the plan
Replacing only water during long, sweaty efforts can drop blood sodium, so events lasting hours often pair fluid with sodium. Sweat rate varies several-fold between people and with the weather, so a fixed schedule can over- or under-shoot.
This is general educational information, not medical advice. Overdrinking can cause hyponatremia, a serious condition, so fluid plans for long events are best refined with a qualified professional.