You can harvest lettuce anytime. Pinch off a leaf or take the whole plant.
I like to thin it to about a foot apart and let the plant get large and make a nice head, then I take the whole plant. Now you get some nice large leaves with good body and flavor.
Black-seeded Simpson, a leaf lettuce.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-