It all depends on the kind of rider you become and how much you ride. More experience=better riding.
I'd say at least a year with a few thousand miles on the 500 before you think about upgrading.
For an experienced rider a 500 tends to be a bit underpowered but is by no means a bad bike. Dont expect it to be the only size you own but dont be suprised if it is.
The thought behind starting small is so that you dont get into trouble. The more power and agility a bike has the easier it is to get into trouble, either a mistake, a situation or just the speed demon in you.