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!