Board Thread:Questions and Answers/@comment-19159073-20130913040547/@comment-3250527-20130913061119

Apparently, it is mentioned in Light Novel 2, Chapter 1 by Silica that the recommended level for clearing a floor was that floor's number + 10. I believe it is right after Kirito says that they have to clear the Hill of Memories on the 47th Floor (And Silica notes her level deficiency; her level is in the low-40s).