You can find the thirst settings in the BP_PlayerInventory then select the Class Defaults button to show the blueprint settings in the Details panel. You will then see the thirst settings like the image below.

Use Thirst – If true the character will use thirst, if you do not want to use the thirst system just untick this option.

Thirst Reduction – (As seen in the image below) Using these settings you can set the amount thirst will reduce while in a character state.

Max Thirst – This is the max amount the thirst value can be set to.

Starting Thirst – This is the thirst value that will be set when a new game is started.

Thirst Reduction Time – This is how often in seconds the Thirst Reduction (As described above) values are subtracted from the current thirst depending on the characters state.

Dehydration Damage – Is the amount of damage the character takes when thirst is at 0.

Dehydration Damage Time – Is how often in seconds the character takes Dehydration Damage.