How much can my elderly father pay me to provide him care in my home where he now is living?

I have been having my father stay with me this past year after several strokes have limited his ability to care for himself. I am now his primary care giver. I have read that he can pay me for this service. He does not qualify for medicaid but has some savings. My question is, what is a reasonable or maximum amount that he can pay me without the government objecting if in the future he does need to go on Medicaid. I understand there are tax implications for me, and that a contract between my father and me would be helpful. Right now, I'm just looking for an amount.
Thank you.