This is my first year harvesting my own garlic, planted last fall. I've spend far too much time google-hunting, trying to figure out when to harvest. Tired of getting interpretive hints (when it's half dry, or a month or two after the scape forms, for instance), I figured it out on my own. Rule of Five. I have five different types of garlic planted, both hard and soft-neck. I have tested and photographed my experiment and as a result, feel very confident my Rule of Five will work for most types of typical garlic bulbs found in U.S. grocery stores. It's possible this may not work with a rarer or specialty garlic, honestly not sure.
Garlic bulbs develop under papery thin skins. Each layer of the sheath on the garlic bulb is expressed above ground as a leaf. When five or more leaves are dried out, that means the garlic bulb in the ground is bulbed out and ready. Count from the bottom of the stalk, at ground level. If you see at least five dried leaf layers, dig down a bit to check the size of the garlic. It's likely to be ready to dig out, hang for a few weeks in a dry location, then add to your favorite dishes.