There are many, many factors affecting your ratios including your credit score, rate and terms and how conservative or liberal your lender may be. Generally speaking, though, your front-end ration (principal, interest, taxes and insurance) should not exceed 28% of your gross income. Your back-end ratio (total debt to income) should not exceed 36%.
Also be aware that many lenders may be willing to loan you amounts that ought to cause you concern. Always be cautious and comfortable with the loan amount and terms. You don't want to become yet another sad statistic, unable to hold onto your home.