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.
I found Rome to be an excellent walking city. Only as a last resort did we hop a bus or take the subway. There’s just too much to see and every turn presents a new and often wonderful vista.