There's a number of good posts from @theoretical explaining how the system works. However probably we just could start with some reasonable assumptions.
If we just monitor how much active whale curators can have now, like
@smooth 2,300,000 SP 2,500 last week
@blocktrades 2,900,000 7,500
@ned 4,500,000 3,500
@berniesanders 2,300,000 4,400

It looks like hardly anyone could manage 20% per year. I've heard top curators could make 60+% though.