If your credit card bills are paid on time but you have some old medical bills on your credit report how much do they effect your credit rating?

The lenders will look at the "overall" picture. What they will do is take into consideration if the medical bills were paid off. If they were not paid off your rating will suffer. If they were paid off, and it has been several years (I will say over 2 years) and all your other debts are paid on time (no 30 days late) and your debt to income ratio is not excessive (not more than say 40% of income) and you do not carry debts that are more than 30% of the available credit (some will go as high as 50%) then you may get a reasonably good score, somewhere around 680 to 700 points.