I've never heard the gallon of medium per month of life. Hmm
How much medium do they need? A cannabis plant is capable of flowering in a very small amount of medium, it just won't flower very much. Some people produce great results using solo cups. I've seen some pretty big plants come out of solo cups used as hempy buckets.
As for your 6"-12" clones, you could use a 2 gallon grow bag, but I suggest using the biggest containers you can. I don't know your strains at all though. If you had a really stretchy sativa then I would think you'll want more medium for it. The worst case scenario with having too much medium is that you have some excess soil and your watering will be a little different. The worst case scenario of not having enough medium is that you lose yield. Its your call.