When is lettuce ready to pick?

I pick lettuce as soon as it starts producing around a dozen palm sized leafs. It will continue to grow more leafs all spring and summer, though when the weather gets hot, the plant tends to bolt. When it bolts and goes to seed, I pull up the whole plant, and replant more. Planting lettuce again in August is good, since many lettuce varieties ( red, romaine, endive, Grand Rapids, etc. ) prefer cooler weather.