With skip-the-line tickets or a guided tour, you need about 1 hour for the Colosseum. We took a tour that included the Arena Floor and Colosseum Underground and were in and out of the Colosseum in about an hour. However, you should count at least 1-1.5 hours for a visit to the Roman Forum and Palatine Hill, plus the queue. So realistically, you need at least 2.5-3 hours for the Colosseum.