It really doesn't matter where you are, garlic is harvested when most of the leaves have gone yellow/brown but about 3-4 leaves are still mostly green. Each leaf is an extension of a wrapper layer around the bulb, so when all the leaves are dead, the wrappers wither away leaving the cloves exposed. This shortens storage life. If you plan on storing the garlic for a few months, better to leave some wrapper layers around the bulb, which means leave some green leaves.

TomNJ

I harvest when there are 3 green leaves left. I am in zone 7 and my early garlics are already harvested but the main crop is still a couple of weeks away. You probably want to dig up a couple of the ones with all brown leaves. This way you can see how big the bulbs are. I suspect those are definitely ready to be dug up.

Another way to tell is to count 20 days from when you cut the bulk of your scapes. By then the bottom leaves should be yellow and the top ones still green.