How far ahead to book?

Be aware of the following holidays in Italy (in addition to the weekends):
April 25-National Liberation Day
May 1-Labor Day
After Mon May 4, in Rome you can find something any time even the same day. Sorrento will be busy on weekends in May, so if you travel there on weekends, book early.
I think fast trains can be booked up to 120 days ahead. However if you are not seeking a discounted ticket, seats don't sell out and you can find tickets at full price even on the same day. Remember discounted tickets are not refundable if you miss the train.

I just finished booking hotels/b&bs for a 4 week trip to Italy in May of this year. With a 10 May arrival in Rome I was beginning to find some of the hotels in Rome and Sorrento were already booked for the days we wanted. This was also true for the other cites/towns we are going to. We were able to find good rooms in all cities/towns but we were not able to get some of our first choices. For 2015, I would recommend you start booking rooms in early January if you want to be sure to get your first choice. For the trains, I wanted to booked several trips at once and found that the lowest fares were not available on some of the earlier trips. So, you need to pay close attention to the 120 days in advance if you want the low fares.

When I first traveled to Italy about 5 years ago, many smaller hotels and B&B's weren't accepting reservations more than 3 months ahead. Now I had trouble finding places for my trip that starts in 3 months. So I'd start looking 5 months ahead. Rome will be easier than Sorrento.

I would consider looking earlier than Bob. I am traveling in May 2014 and found/booked most of our places in September and October. This wasn't completely necessary, but it did give us first choice. I would start looking in November or December rather than waiting until January. Some places will not book that far in advance, but some will - and some will give you a discount for being 6 months out.

I am traveling to Rome in June. During the two weeks or so I spent looking at several different hotel options, at least two of them filled up. By that I mean, when I checked for a vacancy and price, it gave me a response at first and then showed nothing available about 10 days later. All this took place the first two weeks of January.

If you have any confidence in your dates, I would go ahead and think about making hotel reservations. If you have a LOT of confidence in your dates, you can find some pretty nice deals if you prepay.

Thank you all for taking the time to respond. First hand experience is so helpful.