I've been making my own yogurt for a while – 1-2 quarts of whole milk, 12 hours cooked, into 1-2 pints after straining it all day. A few batches ago I started saving the whey. Today I tried to make ricotta from it: I heated the whey (one big pickle jar, probably about 24oz but I didn't measure, plus the extra whey from today's one-quart batch). Heated it on the stove to 195 F, added vinegar, stirred it, left it for a while, no visible curdling happened. Poured it through a strainer with a lot of layers of cheesecloth on it, the liquid that came through was about as cloudy as it was at the start and there were no curds I could see.