A character needs about 1 pound of food and 1 gallon of water per day — 2 gallons in hot climates. One ration covers the food; water usually has to be found or carried, so hot, dry travel is where the survival rules bite.
Going without food: you can last a number of days equal to 3 + your Constitution modifier (minimum 1) before it hurts. After that, each further day without food gives you one level of exhaustion, and eating again only resets the clock once you’ve had at least half a pound in a day.
Going without water is harsher. On any day you drink less than half the water you need, you make a Constitution saving throw (DC 15, climbing on consecutive dry days) or gain a level of exhaustion — two levels if you were already exhausted. Dehydration stacks fast, which is what makes desert and ocean travel dangerous.