There are so many hundred varities of lettuce but let's separate them into just head lettuce and leaf lettuce. Leaf lettuce can be picked at just about any stage of maturity from very young and tender to old and bitter depending upon one's taste and circumstances. New leaves will emerge after cutting off a few from each plant. Head lettuce should be left to form a more mature head and picking the head when mature pretty much forever ends your relationship with that particular plant. You can also plant another crop in late summer after the worst of the heat passes and then enjoy an autumn crop of greens. Slugs, bugs, and bunnies like lettuce as much as folks do so be watchful and wary. I grow mine under a floating row cover which makes life a lot easier, critters more frustrated, and the lettuce much nicer.