Really depends on the Cache, There are a few caches that get allot of DNF's because they are just that hard. I know I logged 3 DNF's on Old 470 before I found it and then there are some of Terry's and Gobler's caches that I know many struggle with. Allot of it does reside with the cache owner to determine what the amount of DNF's need to be done on a cache as they are the ones that know just how difficult it could be.