Whenever I use to make cannabutter, I would heat it all together for no less then 12 hours, on a very low heat, just enough to bring the water to a simmer.

I have also done it for longer then the 12 hours, and while you may think its a waste, it did make a difference according to everyone that ate those edibles. You should also throw in some soy lecithin, as it aids the absorption of the cannabinoids.