I would think 8-10" would be fine. Garlic has a rather limited root system. I'll be interested to hear how it does for you. Around here (NW Indiana) garlic will begin to bulb in about a month and the early varieties will be ready by the end of June.

I'll post back. I read that they're very deep rooters, all the way up to very shallow. So I'll try what I've got, which is 10". I'll let you know. :)

I do not have the answer to your question and like you, I am kind of new to this and search for answers. I did however take a bulb of garlic from my kitchen, seperated it. Left the skins on and planted it into a grow bag. I seen someone do this to potatoes so I thought I would try it. So far so good. I just add some dirt as they grow bigger and now Im near the top of unrolling my grow bag. I sure wish I knew how to tell when they are done though!
Ill post a link to my youtube channel but I dont have a video on just this topic, I have random garden updates mostly.

Im in zone 9b central florida

Sherry

Here is a link that might be useful: My youtube garden updates

Let me preface my answer by: I grow garlic in the north, way northern WI....so I don't have southern tips:)

We do grow about 1200 head of garlic, so we have this pretty down pat for up north. No planting in the spring, fall only. Now I suppose if we wanted garlic greens we could try planting in the summer...but we have way too much else to plant in the spring!!

We have never grown in containers....thought about it when we had little to no room, but never tried it. I cannot imagine sinking the cloves down 18"!! We put our cloves down 4-6", why the need to go down 10-18" when in a container? Our typical spacing in ground is around 8" x 10" making sure to put similar growth habits near one another. For us putting the softnecks next to hardnecks just creates competition issues. The softnecks are much smaller and daintier plants than the hardnecks.

Garlic will be ready for us late June and into July. The leaves on the plants start to yellow, from the bottom up. You DO NOT want to wait till all the leaves are dead...this is too late to dig bulbs. You want about 1/3 of the leaves yellow...or a bit more. Experience is the best indicator. We have pulled bulbs when 2/3 of the leaves are yellow and wished we had pulled sooner. As I said experience is the best indicator...some varities you need to pull sooner, some can stay in the ground later. But, if you don't have yellowing leaves...the plants are not ready to be harvested. And DON'T keep watering when you see yellow leaves, the plants don't need increased water at this time....just the opposite.