How to trim lettuce
The great thing about lettuce is that after you cut all of the leaves off to eat, it grows more so you can keep making salads with a small investment in some seeds and the proper cutting technique.
When the leaves are about 3" long, they're ready to harvest. Clean a pair of scissors to make sure you're not transferring any bacteria to the plant. Pull all of the leaves into one bunch and cut about 1/2" - 3/4" above the soil. Do this with each of your plants until you have enough for a salad. I cut half of my lettuce leaves on Friday of last week and the second half on Sunday. You can already see the new leaves sprouting and it's only Tuesday!