It is best to keep lettuce refrigerated if not using the lettuce the same day.
When you store the lettuce in the refrigerator the lettuce will keep longer than it would when outside of the refrigerator.
Lettuce doesn't spoil as easily as other foods when out of the refrigerator because lettuce grows outside and stays outside until it's picked.
So you could leave your lettuce out at room temperature for a few days to a week at most before it goes bad.
But I always like to refrigerate any lettuce that I'm not using the same day so that it keeps longer and having cold crispy lettuce is much better especially in salads and sandwiches.