Knowing when to lift garlic can be a tricky proposition - harvest them too early and the bulbs will be too small, but harvest too late and the bulbs will begin to lose their quality. The old rule is to sow garlic before the shortest day of the year (Dec 21st) and harvest before the longest (June 21st). Some people also do a spring sowing which won't be ready until late July or August.
A good general rule of thumb is to do a test when a third of the leaves on each plant are gone brown. Carefully push back the soil around one plant and have a look at the bulb to check its size. If it's too small, put the soil back around it. Lift all your garlic when a half to two-thirds of the leaves are gone brown. Lift carefully and dry on racks in sun (or indoors in wet weather) for two weeks. Hang in plaits.