Depending on the car, about a cup or two each year is typical to lose. My Outback loses about that much, typically more in the summer than the winter. I've only topped off the E350 twice in the 15 months that I've owned it. Both times, only a few ounces.
What do you mean by sealed system? I haven't seen a car or truck made in the last 50 years with a sealed cooling system. They all have overflow tanks, and those overflow tanks all have an overflow tube, that is vented to the outside.If it is sealed and you are seeing the level drop that much within a week ,you either have a leak or the car is burning coolant.