Yogurt can be left to incubate safely for up to 12-14 hours. Although it will not spoil, it may however, be tarter than you would prefer. The longer it incubates, the more tart it will taste.

The most common reasons for yogurt not setting are: – Yogurt starter needs to be replaced as it is either old or does not contain sufficient live cultures. – The milk temperature was too high when the starter was added, thereby killing the live cultures in the starter