How long should a cocktail reception last?

My fiancé and i are planning a cocktail style wedding reception and are really worried that people might get bored! Not sure how much small talk people can really make?! We will have a dance floor, plus a guest book to sign, 'marriage advice' to give/wedlibs that sort of thing - as well as having cake cutting and first dance etc - but really how long is safe? We are currently thinking 4 hours..... We just don't want out guests to be bored!!!!