Every lender has specific and different guidelines for the amount of allowable percentage of debt to income ratio. During the boom times of 2004, lenders would allow Debt to Income Ratio's to be as high as 60%. 

FHA guidelines, which are the primary means of qualification in today's market, typically want to see your debt to income ratio fall below 40%.

For more on how to calculate DTI and the difference between front end and back end DTI, check here.