Growing cos lettuce

In Autumn I planted 6 cos lettuce seedlings as I love cos lettuce, especially in a caesar salad. They have grown really well in a position where they get the winter sun. There is something I discovered which has increased their economic value in my eyes. I found that if I removed the leaves without pulling out the stump and roots that they grew a new lettuce head from the old stump. Consequently I now have more cos lettuce than I can keep up with. I recently gave some large leaves to a friend who has a rabbit. I now know that I can pick the lettuce leaves while they are still relatively small (I prefer them that way rather than too big) and still have some left for another meal. My only question is, when do they die off? Do they die off in Summer? Mine are still looking good as we have moved into Spring. I am wondering if I should let one go to seed the same as I do for the other variety of lettuce that I have growing in my garden. With the other lettuce, I planted it years ago and ever since then I let it go to seed and just watch to see where in the garden it will pop up next. Since the first planting I have never had to plant it myself, it self seeds/germinates around the garden! I'm hoping to do the same with my cos lettuce.