How much of what you earn belongs to me?

Speedmaster, at the Pretense of Knowledge blog posted this great quote from Walter Williams:

What’s “just” has been debated for centuries, but let me offer you my definition of social justice: I keep what I earn and you keep what you earn. Do you disagree? Well then, tell me how much of what I earn “belongs” to you — and why?
– Walter Williams

I’ll go one further.  How much of what you earn belongs to me?