An adventurer needs 1 lb (1/2 kg) of food and 1 gallon (3.75L) of water per day.  

If one of those requirements are not met, the character earns a level of exhaustion.

Food and drink can consist of anything from rations, all the way up to a Kings feast. 

The cost of this food and drink is included in the ongoing living expenses of the campaign. 

Pro- Tip: there is no need to lay out the details of eating and drinking in game, unless it furthers the storyline. Leave those necessities for downtime in between sessions.