There are two methods of gathering salad greens. You can use scissors to cut everything about 2-5cm (1-2") from the ground, when the plants are about 10-15cm (3-4") tall. Or, you can pick individual leaves as they're needed. The first cutting may contain more brassicas than lettuces (arugula, mizuna) but if you cut the mix back when the leaves are still small, the lettuce will catch up. The salad greens will regrow for a second harvest in another 2 or 3 weeks.