I think your process is a great idea since it allows for preservation of the weed (by way of the GD). As far as the cooking time, I think your guess is as good as mine.

On another thought, one way to get an airtight environment would be to use a pressure cooker. A pressure cooker is essentially the same thing as an autoclave (which was referenced in the previous recipe). A practical problem is that I can't seen to find a pressure cooker smaller than 4 quarts. Obviously if we are only making a few ounces, there will be quite a lot of air in the pressure cooker with the glycerine/cannabis mixture.

I did find a reference to a lady in the northwest U.S. (Oregon I think) that made the Glycerine tincture using a crockpot over several days. A crockpot is obviously not airtight but she claimed that it worked. I will try and find that recipe again and repost it here.