I grow salad mix for our
local farmers market in the manner Jennifer describes. I can usually harvest from a bed for 3 or 4 weeks. After that the quality declines, particularly texture get tough, even though it is only hot
enough for quick bolting to be an issue in June. I transplant into 35 square feet of bed every two weeks--three or four lettuce plants in a 1.25"x1.25" cell (72 cells per 10"x20" flat) every six inches. I thin them to one plant when they are the height you're aiming for and get maybe 3 or 4 ten ounce bags of salad mix. I figure about 3 or 4 medium salads per bag or per 10 square feet. If you are going to harvest everything at the 4" stage you could plant much denser. The week after thinning I start harvesting the biggest leaves and get another 3 or 4 bags. The next two weeks I get double that and then the quality starts dropping and I usually take the newer leaves for a much smaller harvest. You get more yield per square foot this way, but it is more labor intensive than clear cutting at the height you want. I agree with Jennifer's estimate for the technique we use, but think you may need twice that space if you clear cut.
ps. I like my lettuce leaves a little bigger than the bland, tender/insubstantial 4 inch meclun stage.
I'm in the foothills of the San Pedro Mountains in northern New Mexico--at 7600' with about 15" of precipitation, zone 4b historically--growing vegetables for the local farmer's market, working at season-extension, looking to use more permaculture techniques and join with other people around here to start and grow for farmers markets.