Cut and come again salad leaves can be harvested as required when the leaves are roughly finger-length.
Lettuce plants should be harvested when the heart is firm. Test by pressing the top of the plant gently with your hand, rather than squeezing.
Cut the plants at the base rather than pulling up. Discard the root and lower leaves.
If the heart begins to grow upwards, the plant is bolting and preparing to seed. Harvest that lettuce immediately and eat it – it will become bitter if left in the ground.