There is a recent thread on this:
http://www.wgt.com/forums/p/48159/280125.aspx#280125
where some folks are trying to get a precise list for all levels.
In theory there is no fixed upper limit, but the curve is very steep. In the mid levels you can advance about 1 level per day with moderate play (e.g. 1 18-hole tournament round per day), but at the higher levels apparently it could be equivalent to even a year's worth of such play to go up one level.
In the 80s, where I am, it is a few weeks of moderate play to advance one level.
The highest player is at level 100 - could be there a long time!