This ring is ilvl 17, your best bet is farming monster level 17-19 monsters... That is definitely a normal difficulty, somewhere around Act II (if you can find a monster level by acts somewhere in the interweb, you have your answer there).
EDIT: By the way, thanks for pointing out this ring for me, looks very promising for the new Paragon system. However, I dunno if it is worth farming... But Blizzard will make legendaries below ilvl 50 drop more often (by what factor is unknown), so farming with high level character with max magic find seems plausible.
Thanks. I was wondering why BLZ don't add some ilvl 60+ rings with such EXP bonus. Despite the 20%+ Exp bonus, this ring is a crap.