The busiest season for tourists in Rome is the spring and early summer. The weather is warm (but not yet too hot), and you avoid the worst of the winter rain. However, the city can really fill up with tourists during this time, so it's a trade off: if you come really off-season, in mid-winter, you have emptier streets, but there's more chance of rain and cold.
Another thing to take into account when you're planning a trip is Rome's many events and festivals. For more details of these, take a look at our section on annual events in Rome.
Summer in Rome
Depending on where you're coming from and what sort of weather you're used to, it might be worth avoiding the heat of the Roman summer. From mid July into early September it can get unpleasantly hot, with temperatures reaching into the forties (Celsius) or well over 100ºF. It's also worth bearing in mind that half of Rome shuts down for August, as the locals flee to the seaside or sit in their apartments nuzzling the air conditioning.
If you can stand the heat, August does offer the advantage of emptier streets, but if this is your only trip to Rome, closed shops and attractions can be frustrating.
How long to spend in Rome
There's no danger of exhausting Rome in two weeks, or for that matter in two years. There is so much for tourists to do and see in Rome that you simply aren't going to get through it all. In fact, it's a bad idea to try to see everything, as you may end up focussing more on what you've missed than on what you've enjoyed. (For example, if you're only going to be in Rome for one or two days, you may want to skip the Vatican Museums and the Sistine Chapel, or you could end up spending most of your time in Rome experiencing a real Roman queue.)
A little time spent researching the things you particularly don't want to miss will pay off when you're there. Work out what you really must see, and treat anything else you might find time to look at as a bonus. After you arrive, be prepared to take things at your own pace. You could charge around trying to see everything, but then you would miss the pleasure of simply being at a loose end in one of the most beautiful cities in the world.