Curveman:
In my experience, 6dl (600ml) per day is do-able. That would be around 2500 calories, so on the basis of 3500 excess cals per pound of fat gained, that would be around 5 lbs a week gain. I doubt in reality it would be that much because not all the cream gets digested. However if you spread it out through the day, that might improve digestability.
You will probably have a few days where you don't get the full 6dl in so I would guess around 3 months to gain 30 lbs.
fatchance:
Remember, that as you gain, the number of calories to maintain that gain rate increase as well. Probably 12 to 15 calories a day per lb.
So unless you keep increasing your intake, your gain RATE will slow as you gain. Of course, as you get comfortable with your increased consumption, that may indeed happen on it's own, or may not.
I have noticed though, that unless you monitor it and correct, drinking the cream will sometimes cause you to eat less especially in the beginning.
I found that I usually drink it right before bed. So I have normal meals (doubling dinner sometimes) and snacks and whatnot throughout the day, and then right before bed drink heavy cream so it doesn’t affect my real food intake.
I’ve finally been able to put on weight like this, albiet not as quickly as I want to, but it seems to be working at around 2 pounds a week to two weeks so far.