Lay a piece of plastic wrap directly on the surface of the demi's and it will help seal them and keep air from getting to them and oxidizing them. I'd agree 30 days is about the life expectancy, but smell, smell, smell when you want to use them, and if it smells off, then don't ! Better safe than sorry. Or portion it out into smaller quantites of 1 or 2 tablespoons (put them on a parchment or wax-paper lined sheet pan), freeze until solid, then toss into a zip-bag and freeze until you need it.

Refrigerated your demi should keep for at least thirty days and probably longer. That's kind of it's whole point. It's concentrated to be there when you need it.