Here is a healthy garlic plant. It sports 10 leaves with a scape on top. (that little brown leaf on the left counts) When 4 - 6 of these leaves turn brown, it is time to harvest. Each leaf is a wrapper for the bulb underground and so you want 4-6 green leaves to ensure a tight wrap on the bulb after curing.
I look at my garlic everyday this time of year marveling at the differences among the scapes. Here is a classic rocambole curly scape. The purple stripes like Pskem dip their heads like shy swans atop their plantSo I have harvested at least half of the scapes now and am storing them in the fridge. They make great stir fry and are wonderful additions to anything on the grill. You want to pick them once they have curled, but BEFORE they begin to straighten. It will send more energy to the bulb as these wonderful plants turn from scallions to full bulbs in less than a month.