From my experience, I was on about 62.7 last night. Shot a couple of good rounds, which brought my average down to 61.2, and that was enough to bump me up to Legend. I believe it is different for everyone though, as in there is not a particular average, which after beating, you move up to Legend.
However, a couple of things you should know: when you get there, each hole will be about 30 yards longer, and you will be playing approach shots to a lot of holes with woods or 3 irons (Kiawah back 9). Also anything not hit perfectly is going to go off target more than before.