How to play rule #1: The DM Describes the environment.
The DM decides what is hot. The DM decides if the characters need to consume extra water. And the DM will tell you these things. From the DMG:
You can pick weather to fit your campaign or roll on the Weather table to determine the weather for a given day, adjusting for the terrain and season as appropriate.
Extreme heat is 100°F or more.
The DMG states:
When the temperature is at or above 100 degrees Fahrenheit, a creature exposed to the heat and without access to drinkable water must succeed on a Constitution saving throw at the end of each hour or gain one level of exhaustion. The DC is 5 for the first hour and increases by 1 for each additional hour. Creatures wearing medium or heavy armor, or who are clad in heavy clothing, have disadvantage on the saving throw. Creatures with resistance or immunity to fire damage automatically succeed on the saving throw, as do creatures naturally adapted to hot climates.
That said, the point at which additional water must be consumed is left undefined.
When in doubt, draw from your real world experience.
The rules of the game leave a lot of stuff undefined, and here I've just told you to make something up. What I like to do is reflect on my real world experiences for things like this. I know, D&D is not a
physics meteorology simulator, but this is a place where real world experience can be helpful.
Think about your experience - at what point was it hot enough outside for your thirst to increase? Make a judgment based on your experience. For me, it happens in the mid 80s when it is relatively humid.