One of the best things I did was visit St Peters when it opens at 7am. The rest of touristic Rome is having breakfast. The hordes arrive later. There are small masses taking place in side chapels but you pretty much have the church to yourself or very few at least. You have to get up early of course but I got to view the morning sun streaming into the church from the front and that was also quite glorious.
A taxi driver wanted 25 euros to take me from St. Peters to the Colosseum. I looked at the map and told him I would pay 5. We were unable to bridge the gap, so I walked; it took about 40 minutes, there being interesting buildings and ruins along the way. I'm an old hand at coverning Civil War battlefields on foot and at speed, so I can cover a fair amount of ground. Having no interest in shopping and not needing to eat helped as well. But I could still have used a bit more time.
About a month would probably suffice.