I made mashed potatoes with sour cream and cream cheese that have been in an airtight container in my refrigerator for a week. Are they safe to eat?

As long as they look and smell fine—meaning the color is not grey and the sour cream and cream cheese have not curdled—you should be fine. Bear in mind that while restaurants keep food product for that duration, their refrigeration is controlled. Home refrigerators can fluctuate in temperature more often, which can lead to a shorter shelf life.