Preparation and cooking time isn't listed because it depends on several factors. For example, whether (cooked and mashed) potatoes are left in the fridge overnight, how long it takes one to make the dough, how big the boiling pot is (determines how many dumplings are in at any one time), etc. Each time will be a little shorter, but keep in mind that overall plum dumplings do take a considerable amount of time.
A serving size is about 2 dumplings, thus this amount of ingredients yields about 15 servings.
If you have some dough left over, you can form it into gnocchi (as the dough is pretty much the same), place in a freezing bag, freeze, and then make gnocchi later.
Since plum dumplings take time to make, you can double the ingredients and make a big batch and then freeze a portion. You freeze them after they're formed into dumplings, but before they're cooked. Once you're ready to make them, boil water, and throw them in frozen. Then follow the rest of the steps.