If you've ever wondered how tall a lettuce plant would get if you never got around to harvesting it, I can tell you. I was considering harvesting seed from the two giants pictured below, but I removed them to make some garden chores easier. They are as tall as our young cherry tree, which is probably about four and a half feet tall.
Lettuce going to seed
Also, in case you are interested, two very tall lettuce plants make fantastic, if short-lived, swords for duelling. En guarde!