When to harvest hardneck Garlic?

I got a couple of bulbs last Autumn and planted the cloves in big pots. They spent the winter in my unheated greenhouse and are now outside. It's been incredibly hot recently which no doubt will have sped them on and scapes have appeared (now eaten and delicious they were too!), a couple of the outer leaves are going brown and the plants are about 3ft odd tall. Any idea how long after the scapes have appeared is the best to harvest? I think you shouldn't wait until all the leaves have gone brown but not sure when is the optimum time from a good size/storage ability point of view. Thanks.

 

I always let mine (softneck) die back pretty much.