Homemade yogurt can be used multiple times to culture a new batch of homemade yogurt. After 3 or 4 times of culturing yogurt with a homemade yogurt culture it will start to weaken. When you notice a batch that doesn't get as thick as previous batches use a unflavored plain store bought yogurt or buy a yogurt starter for your next batch. Live yogurt starters will last about as twice as long as your homemade yogurt culture.