Don’t know if I’m misunderstanding Jay.  As far as I know, you have to scald the milk each time, not just use ‘fresh’ milk, so alien bacteria and possibly enzymes don’t interfere with the yogurt bacteria.  But yes, you can continue to use your own culture for as long as it works.  Unlimited supply…

Sometimes it get contaminated and you have to get new culture from the store–or someone else.  My only problem has been that using older culture, it sometimes gets a “gooey” consistency like okra. Nothing wrong with the taste.

However, if you let it ‘work’ too long before popping in the fridge, it will get quite sour instead of the initial creamy taste.  And sometimes separate a little.  BUT this is how many poorer countries keep their milk without refrigeration.  In India, they mix chopped raw onions and spices with it and use it as a side dish during meals–call it “raita”.  They also use yogurt, watered down and called ‘buttermilk’, on plain rice for a ‘poor man’s meal.”