I say the minimum is an hour a day (with 30 minutes being the minimum session time).
I personally do 30-45 mins in the morning and 2+ hours in the evening. On the weekends, I'm trying to ramp up to 5+ hours. To me, it is about time under the domes -- otherwise, how will you trigger growth? Swelling isn't going to do it; it is the pulling of tissue that does it. (I'm pumping now)
At some point, I will probably have to start skipping days but I think that's still a few months away.
DJ