I never stop planting lettuce. I've got romaine and another variety growing right now. They don't last quite as long as during the cooler months, but they still do fine. I have mine in an area that gets afternoon shade to avoid the intense heat. Oh, I'm in zone 5.
I'm in zone 5 and put a batch in on Friday. Also peas.
And I'm being terribly brave and attempting cucumber--I'll be happy if I get a vine, thrilled with a few flowers, and estatic if I get one bitty fruit. If I actually get a decent harvest, you'll probably know by the faint "yippee!!!!" you hear coming over the horizon.