I think I ruined a batch of yogurt

I made a batch of yogurt yesterday afternoon, not really paying attention to the time. I checked it after 7 hours of incubation, before I went to bed, and it still seemed thin. I figured, well, I almost always seem to wake up at midnight since we’ve moved, I’ll just pop it in the fridge then. But alas, last night I slept all the way from 10:30 until 6 this morning–and my yogurt incubated from about 3 yesterday afternoon til I woke up. That’s some 15 hours. The cooler was nowhere near 100 degrees by morning, and the yogurt still seems thin, to boot. Some websites I’ve checked simply say that longer incubation times yield tangier yogurts; another one said that as the temperature changes and time marches on, it becomes ripe for mold. I’m not sure what to do. If I toss it, I’ll lose 3 pints of yogurt, worth about $1.30 in terms of milk, and I’ll have to start over with a new culture (ie, new cup of plain yogurt, rather than my own). I’ll probably do that–better safe than sorry, I suppose. What a bummer, though I am glad for the sleep.