How many days would you spend in Rome?

Yes, I agree with Kent. 3 is fine as long as you make a list of what you want to see and be realistic. Remember that the Vatican museum is so large that it would take 4 days to see whole thing, so don't plan on seeing everything. Just get it down to the "must see" items and then do the rest if there is time.

Depends on what you want to see and the pace you can sustain. If you want to hit "all" the sites, you probably need 3 days. If you only want to hit the biggies: Colosseum/Forum/Pantheon, Vatican/St. Peter's, Borghese, then 2 days should do it, if you get your planning done in advance. In June, the lines/crowds can give you trouble, so take advantage of the suggestions in Rick's books to avoid them as much as you can.

Three full days minimum is good. One for Vatican Museum & St. Peters. Another for Colosseum, Forum and an evening stroll. A third for other sites and to enjoy yourselves.

Even then, you'll regret not having spent more time in Rome and another week in Europe.

Jill, I normally suggest a minimum of five days, however as the others have mentioned the time will depend on exactly what you want to see. I believe Rick has suggestions in his Italy book for one/two/three day tours of Rome. You might get some ideas by looking those over.

I found there were so many sights I wanted to see, I ended up "overdoing" the touring and had to take a bit of a break.

From one Jill to another,

On our last trip we had 3 full days and 3 nights in Rome. We felt it was a little too much. Rome can be exhausting due to the crowds and the pace. I think 2 days is perfect. I was reading the recent ETBD best of europe and I believe Rick says Rome is best done quickly. I totally agree! If you want 3 full days you might do 2 when you arive and then 1 before you depart.

I agree with Jill, Two days in Rome is perfect.

That's what we did last year, with no regrets.

You can see and do, so much more in other parts of Italy

In November we spent 6 nights in Rome. This was too much. We walked everywhere and felt like we finished Rick's Rome 2008 book (there were other things left undone, but we didn't have any desire to do them even though we had a full day to kill at the end). The most we should have spent was 4 nights. I would have made a case for 3 nights, a late afternoon trip into Tuscany for a couple of nights and then adding an extra night to our Venice stay.
I would say to add 3 days to your arrival/departure.

Just to speak for the Rome lovers out there, I've taken two trips to Rome, one for nine days in the summer and one for 10 days in the winter and I left Rome both times desperate for more. However, I study ancient history and so, I have to see every last surviving brick of every last building. I'd at least give it three days so you can spend one just wandering around and feeling the place without having to do the Caesar shuffle.