This is really dependent on your own travel style, the country you're in, and what you're doing every day! Most countries will take card payments for dinners or lunches, but it's always smart to have some local cash. A good rule of thumb would be to bring the equivalent of $40 USD worth of local currency per person per day as a backup in case something happens with your cards or the business does not accept cards. You can always exchange unused cash at the airport on your way home. Obviously, it would be up to you to increase or decrease that amount based on whether you like to shop for souvenirs from other countries, if you prefer a nicer, more expensive meal, and what country you're in and how far the cash will go! Most places will also have ATMs available if you're staying in a developed city or town (excludes Refugio's and some very small towns).