Between my mother and I over the decades we have made three different things:3 days...? Never heard of doing it this way. Not saying it won't work; mostly I can't think of a place in my house that is a good warm place for milk to sit for 3 days.And not get spilled.
I make yogurt and this is what I do to make 1 gallon. Heat milk to 180 degrees F. Cool to 120 degrees. Pour into quart jars, putting about 1 cup into a bowl with some plain yogurt.* Mix the milk/yogurt thoroughly, and divide this mixture into the quart jars of plain milk.
Put it in a warm place for 5-8 hours. My warm place is in a cardboard box lined with towels. The warmth of the milk is enough to culture the yogurt.
I strain out some of the whey (save this) for a thick "Greek" style yogurt, and mix the yogurt with honey. I end up with about a half gallon. I use the whey for making bread, mixing with the chickens' feed, cooking rice, or in a smoothie.
*What plain yogurt to use? I buy a small container of Greek yogurt at the store as my starter culture. I have found that different brands make the yogurt taste different. You can also buy a powdered culture, but I never have. I use the entire container (6 oz, I think), which might be more than necessary, but it works, so that's what I do.
I tried making yogurt with my yogurt, and it didn't culture/set up very well. Maybe if I'd let it sit overnight...? But I just did what I usually do. I'll buy some next time.
A couple other things... I use fat-free milk, and it's plenty creamy. Also, don't use "ultra pasteurized" milk.
1) Yoghurt - made almost exactly as you describe above though we left it overnight rather than 5 hours
2) Something akin to Swedish Filmjölk which is a lot more sour than yoghurt but is basically the same sort of thing and made the same way but from memory it took a couple of days - I think the longer it sits the more sour it gets. When my mother was into that we had an 'airing cupboard' where the hot water tank was in a cupboard and the Filmjölk sat in there
3) I now make Kefir which is a more complex culture than is in yoghurt - it has more different bacteria and fungi and lord knows what. That doesn't need to be warm - I keep it on the kitchen counter. The mildest version gets made in a day, leave it a couple of days and the curds really separate from the whey and it gets eye wateringly sour!
And that reminds me, my little Kefir grains have already used up their milk and need feeding. Back later.
Oh - and chickens love Kefir even when very sour.
