We generally consider something to be “fully dehydrated” if >95% of the moisture is removed, which is determined non-scientifically by look and feel. However, you can also “dry” but not “fully dehydrate” some items for a different desired texture. For example, when you go to the dried fruit aisle at your local grocery store you may notice that the “dry” items are still moist to the touch. They may be labeled as “dehydrated,” but if they are moist we would not consider that truly dehydrated.
If you want your mandarins to be soft and somewhat moist (like the consistency of a raisin) you can dehydrate for a shorter period of time at the same temperature; approximately 8 hours.
When completely dehydrated, these will last for years, but if only partially dried they should be eaten within a few weeks.