Homemade Yogurt Recipe | Serious Eats
How long it takes the yogurt to set will depend on the temperature at which it is held. This can be as short as 3 or 4 hours and as long as 18 hours. Once the yogurt has set, allow it to sit out at room temperature for up to an additional 12 hours to ensure a strong culture; if you’re working with a culture you know well, you may not need to let it sit out for so long, especially if you don’t want it to grow too sour. There’s no one good rule here except to give the yogurt the time it needs to sour and thicken properly.