There isn't a specific time line. The freshwater conditions are not ideal, and will result in problems, IME, evnetually. Probably not tomorrow, but within a year. This is why you should take them back--you don't know when you will be getting a new tank. There will always be mollies available for purchase, so it makes more sense to return them now, and buy them when you can provide for their needs.
Adding salt to your tank is not an option, and will cause serious problems for the angel.