Gaining

I’m currently about 170lbs and athletic (abs, etc.)

If I were to drink a quart of heavy cream (mixing with soda or something), on top of my regular maintenance calorie intake and activity level every day for a month, what could I expect?

It's tough to say. Everybody is different, and we don't all gain at a consistent rate. Plateaus are also an issue.

In my case, a quart would be about 75% of the heavy whipping cream that I normally drink per day during a gaining cycle. On average, I gain about 20 pounds over the course of 4 weeks, so about 5 pounds per week.

If I reduced my intake to one quart per day and drank that for a month, I think I would gain about 16 pounds. Heavy whipping cream is very effective.

A couple of years ago I did an experiment with a pint of cream a night for 6 weeks and just eating normally during the day and went from 170-190 so expect some serious gains

My quick math says there's about 3284 calories per quart, which is close ~0.94 of 3500 calories, which is the amount of excess calories that go into 1 lb of body fat. So that would be the upper limit of how much you'd gain in a day, which is on the upper end of some of the famous/rapid gains.

But most people's digestive systems probably aren't efficient enough to metabolize all that fat, especially if consumed all at once. I can guess at a few things that might help with digestion/absorption, though I'm not sure (and this definitely doesn't constitute medical advice):
1. take a digestive enzyme with lipase (lipase is the type of enzyme that breaks down fats for absorption, and one of the limits to the digestion of fats is how much lipase your body can muster up for the process)
2. Space out the consumption of the quart
3. Add something sweet (I'd probably go with syrup. Personally I don't think soda and that much cream would sit well for me, but if it works for you, have fun). I learned from another user here that insulin (which will be released when you eat something sweet) cues fat cells to absorb fats and proteins (in addition to blood sugar, which was the only function I knew about before)
4. Drink it in the evening (I find it very filling, so if I have it early in the day, I just consume fewer calories throughout the day overall, since I'm not as hunger. End result is that at the end of the day, the cream isn't all excess calories)

To avoid being gross... your experiences on the porcelain throne will hint at how much fat you're actually absorbing