As much money as they choose to make. There should be no ceiling on anyone's earnings.
Besides, has this class warfare really helped the middle class?
The rich laugh their asses off at high income taxes, and the poor get refunds, but the middle class, we are the one's that really get shafted by this progressive income tax system.
If you have several million dollars in a bank you don't have to work, you can live off the interest, and thus you can laugh when the government takes a portion of the interest, because there will always be more interest.
Though your argument about the quantity of money. I'm not sure if you are talking about income or wealth. There is a very large difference.
A person working at McDonald's can eventually become a millionaire, if they are diligent in saving money instead of chasing every whim, but it takes them a lot longer than some one working as a lawyer, or a CEO, or a doctor.
But would you trust some one at McDonald's to represent you in a legal case?
Or to do surgery on your spinal column or eyes?
I know I wouldn't, and thus I am willing to pay extra for their services, because I am ultimately paying for the time it took them to go through school.