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Felines are what is known as "true carnivores" . This means they get no nutritional values from anything other than meat/fish.
Cats eat greens as a tool, usually to bring up a furball or something else that's causing stomach upset.
Learn more about a cat's actual nutritional needs (not what the commercials say) here:
Here is a link for more information on possibilities of why else they might eat greens
As for how much lettuce should the cat be allowed to eat? As long as the cat isn't eating it exclusively or excessively, it will probably just cause some vomiting. If the cat goes after this a lot, there might be a reason - and having the kitty checked for intestinal disorder or infection is highly recommended.