Every wedding is different and every bride has different needs. The amount of time you need for photos can vary depending on what day of the week you are getting married, how many guests you are having, what you would like photographed, and the length of the ceremony and reception.
With that said, if you are planning to get married on a Saturday with more than 50 guests, you plan on having a full ceremony and reception, and you would like photography coverage from getting ready ready photos through the end of the reception, you should book at least 8 hours of photography coverage.