I spent a month in Rome this summer. I'd say a weekend is the perfect amount of time, any longer and you realise Italians should be wiped off the space of the planet... Most of the sights are a tad dull unless you have a tour guide telling you some, probably made up, stories. Guide woman in the colloseum made two men in our group fight and likened me to a vestal virgin. Made my life. My top tip is don't bother paying for it, just tag onto a decent sized one and they'll be none the wiser.