We have been eating our own lettuce in salads since the end of March. I have them planted in crates so I could use potting soil and hopefully avoid any bitterness from our alkaline soil.
This crate I planted on March 17th and kept it in the greenhouse at nights. I cheated and used starts from Columbia Grain and Feed instead of starting from seed. It has been harvested from several times, but is still going strong.
This crate has another pony pack of starts I bought several weeks later and I transplanted in some that I started from seed.
These are my little baby lettuces from seed. It is a loose leaf variety called Salad Bowl.