4-8 hours 
After 24 to 48 hours, the beneficial bacteria present in the matsoni starter will cause the milk to transform from liquid to a syrupy, semisolid mass and that, dearest real food lovers, is the easiest yogurt you’ll ever make. Incubated at 115°F/46°C, yogurt will coagulate within about three hours, but if left too long it can easily curdle. I prefer to ferment it a bit more slowly at a slightly lower temperature, four to eight hours at a more forgiving 110°F/43°C. Even longer fermentations can yield more tangy flavor and fuller digestion of lactose. I have heard of people fermenting yogurt for as long as 24 hours. At lower temperatures, coagulation will take longer, and the end result will probably not be quite as thick. Even more so, if you ferment it long enough, the bacteria will consume all the lactose, which is a type of sugar in milk that can often cause problems. In the end, homemade yogurt is an amalgam of bacteria, fat and proteins (casein and whey) . Yet, some people might still be worried with the casein content of yogurt. Why not just take some commercial yoghurt and give a further 24hrs fermentation to use up the lactose that is in it? According to Dept. of Agriculture dogma, additional milk solids including lactose and protein are added to commercial yoghurt and then subjected to the short fermentation. Matsoni and other easy, room temperature yogurts require a longer period of fermentation. Where Greek, Bulgarian and other thermophilic yogurts require only eight to twelve hours to culture properly (you can culture them up to 24 hours, if you like), matsoni should be cultured for about 24 hours and up to 48 hours. Traditionally yogurt is fermented for 4-8 hours. The longer it ferments the more beneficial bacteria it will contain. Also the longer it goes the less lactose and more sour it will taste. GAPS and SCD have you ferment yogurt for at least 24 hours to make it virtually lactose free. I won’t lie. Coconut yogurt takes a long time to make. To get a true tangy taste I have found that I need to ferment it for about 24 hours and then you need another 6-8 hours for it to chill and set up in the refrigerator.
http://nourishedkitchen.com/easy-homemade-yogurt-matsoni/ http://www.splendidtable.org/story/yogurt-an-excerpt-from-the-art-of-fermentation http://paleoleap.com/homemade-yogurt/ http://www.breakingtheviciouscycle.info/knowledge_base/detail/yogurt-faq-from-elaine/ http://nourishedkitchen.com/easy-homemade-yogurt-matsoni/ http://divinehealthfromtheinsideout.com/2013/04/guest-post-how-to-make-the-most-healthful-homemade-yogurt/ http://gutsybynature.com/2013/10/18/adventures-fermentation-coconut-milk-yogurt/
how long can you ferment yogurt