Because of the unseasonably hot April weather, the garlic is 2 weeks ahead of schedule. In fact we have already harvested the early kinds: Tzan, Shantung Purple, Uzbek Turban and Thai Purple. These Turban type of garlics usually get harvested in mid-June. The others that we usually start to harvest July 1st look like they'll be ready by about June 12th - 15th.
When half the leaves are brown, including the ones that have fallen off, are brown it is time to harvest. If your garlic is falling over, it's definitely time to harvest, maybe a bit past the ideal time. If you have scapes (only on hardneck garlic) you snap them off when they make a full loop. We have harvested about a third of ours, so far. This gives more energy to bulb-making, so they will be larger. However the ones with scapes left on do store a bit longer.