When to book hotel/Airbnb?

Well, speaking from personal experience in booking rooms for an upcoming trip to Europe, many others are early birds too apart from Europeans. If you know your itinerary and your accommodations are somewhat or more popular...book ASAP.

Airbnb's don't go on promotion so there is never a good reason to wait.
Occasionally some renters don't post the next calendar year until January but I think that is less common now and you are already past that point anyway.

So to answer your question, the answer was yesterday for airbnb's. Cinque Terre especially inventory is low to start with so you by no means booking early if booking today.

Hotels the answer is a little mixed based on what type of hotel you want.
If you want the most desirable places you are already late, the places that typically are ranked 1, 2, 3 for a town on TA are often great places at rates that are worth the experience so will be the first ones to book.

As far as rates, if they are highly ranked and boutique type or family run, there will not be promotions later so best price is early as possible.
Some of the larger establishments and ones maybe without such great reviews and the chains do have promotions and may have a sale on remaining rooms if you wait until much closer. For your time period and locations though, that is a busy time so I wouldn't expect off season type deals and would not wait.

I like Airbnb type places for 3 or more nights, 1 or 2 nights prefer hotels or a hotel like bnb.

Sometimes it costs 10% more or so, but if you can try to make bookings that can be canceled with penalty or minor penalty.
For me at least I sometimes tweak my plans as the date gets closer and like the freedom of being able to change a night or two after my initial bookings are made. It is hard to plan each day out 9 months in advance but I think important to lock in lodging that early.

We don't use AirB&Bs but I wouldn't have any hesitation booking hotels now for high season, especially for the Cinque Terre. Booking early gives us the largest number of choices for the locations/rates that we want, and a better chance of getting into very popular accommodations which fill quickly.

The only caution would be not to get into booking situations you can't get out of should your trip fall apart. I use booking.com a lot for our trips abroad, and while non-refundable rates are lower, I nearly always choose a higher refundable rate (within individual cancellation periods) in case of an unanticipated snag.

Wow, I need to get moving! Thanks everyone! We will start booking this week!

It'll be fun! It's always nice to know - and get excited about - where you'll be laying your head come vacation time!

Yes, getting very excited now that I know we should be booking! :)

You might want 2 nights in Cinque Terre. Part of it's charm is in the mornings and evenings, before and after the tourists (cruise ships and tour buses). One night anywhere doesn't give you much time to see much.

I book as soon as I have an itinerary roughed out. I try to make cancellable reservations as I proceed and until I finalize my itinerary. www.Booking.com is good for having that option. Airbnb, in my experience, usually has very strict cancellation guidelines. I use www.VRBO.com a lot for apartments, and sometimes I contact the apartment owners (whether through VRBO or their own sites) directly to get favorable cancellation terms, i.e., sometimes 6 or 8 months out they will hold an apartment for me for a week or two without a deposit while I finalize the plans.

I usually book about 4-6 mos out - depending on the area and if I find an airbnb that I love. Last year when we were going to be in Amsterdam in April, I booked it in Dec. (As soon as the plane tickets were booked)...but for our stay in Dordrecht, I did it a few months ahead. But I booked London about 3 weeks out - because we had friends we were going to stay with, but they got the dates wrong and weren't going to be home - and I found this out the week before we were leaving. But I found a nice airbnb that I most likely will use again if we can't stay with our friends. I still had lots of choices.

I'd also look at length of stay and your needs (do you need just a room or a whole apartment...do you need something in a popular area...do you need an elevator) - when we stayed in Rome, we needed five nights so I booked it about 5 mos out. But if I only needed a night or two, I leave it a little longer.

Of course, with airbnb you pay as soon as you book, not after. I actually like that, as then that is paid off before we go away, so less expense to pay when we get home. I always like to make sure that they have a half decent cancellation policy (some airbnb's allow no cancellation without a monetary penalty once booked - some allow a few days, other allow changes only within a few months). Booking.com has some cancellable bookings as well.

We did a 28 day driving tour of Wales and England in October. Prior to booking our accommodations, I used several websites like kayak, tripadvisor, booking.com, etc. as well as Airbnb.

The cities that we visited were all small to medium size, but parking was a real issue. We needed to have parking in a place near the city center were we would not have to use the car. We wanted to be able to walk the city or town.

Some of the places we visited were Bath, Cardiff, Tenby, Chipping Campden, Warwick, York, Pickering, Durham, Keswick and Winchester.

In some cities, I found Airbnb apartments that were comparable in price to the B&Bs that we did book. Yes, we would have been able to cook some meals and save, but we never found apartments that were close enough to the city center. That is why we went with B&Bs, Inns or small hotels.