I harvest all lettuce by pinching larger leaves, at any time that they are large enough to suit. I even do this with so called heading lettuce, as you get more dark green leaves that way, and get a larger overall yield IMO.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.
Alex
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.
[img]https://donce.lofthouse.com/jamaica/lettuce_09.jpg[/img]
Black-seeded Simpson, a leaf lettuce.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-
If you want a "head of lettuce" like you buy at the grocery store, most buttercrunch varieties take 72~75 days, but since the weather's getting hotter, you might want to just harvest while you can. Cutting the outer leaves as you need them is the easiest way to get the freshest, tastiest lettuce. You can try to keep your lettuce cooler by misting and watering frequently, as well as by providing some shade, especially from mid-day sun.