More natural than the usual laboratory of chemicals in store bought and, because of the high sugar content, it keeps until used up...probably 1-2 weeks depending upon the temperature of your refrigerator. But, exactly how long? I wouldn't know...I doubt that it will ever last me, or you, long enough to spoil! Also, I have no doubt that it would probably freeze well if you make a large batch.
This recipe uses milk and powdered milk, but there are proportions for using only powdered milk within the directions; as well as alternative flavor suggestions. It started out using a substitute sweetened condensed milk (SCM) idea (you could just flavor a 50/50) blend of SCM & regular milk. Then I said, "Why bother making the SCM substitute first? Let's just make the whole thing at once." To make it easier to remember, I rounded measurements off to the nearest 1/2 cup, and it came out great.