I did 7 weeks last Summer, and will soon be back for a full year : ). I guess the minimum amount of time is pretty much: how much time do you have?
But in all seriousness, it definitely is enough time, it also very much depends on:
- how much you already know: knowing more about the culture will allow you to appreciate things more, and lessen culture-shock.
- what you're interested in: personally I prefer getting off the beaten track and seeing smaller cities, as well as going places where you can interact with the people, not so much as a tourist but as a person.
- how much Japanese you can speak: especially if you want to get off the beaten track, being able to at least ask for directions and understand the answers (which can sometimes be a tad complicated) is super helpful. I didn't know much more than that though.
- When you're going: while planning the itinerary I would very much consider what events are happening where! Also, in fall of course you should consider going somewhere with beautiful fall-scenery.
If you're reasonably confident in your communication skills, I recommend focusing on maybe a bit fewer major cities and going to a few more "small" destinations. Imho, if you don't really take the time to get to know a region and it's people, there won't be *that* big a difference between cities as far apart as Sapporo or Tokyo or Hiroshima. (Then again maybe I'm just a little thick...)
Also, if you're planning on staying so long you'll probably want to plan well how you will travel in this time: unless you're filthy rich, if you just zip from one major-city to the next on a shinkansen it can get very expensive very fast.
A few "off the beaten trail" (at least somewhat) destinations that are near the "major" destinations you listed:
- since you plan on passing through the northern coast to get to Kanazawa, I recommend also the Kiso-Valley trail between Magome and Tsumago.
http://www.japan-guide.com/e/e6076.html- Near Hiroshima is a town called Onomichi: aside from delicious food (which, let's face it, is pervasive of all of Japan...), Onomichi has very very many temples, some not too big, and some tiny. You can enjoy a beautiful walk around there, and then rent a bicycle and cycle down the Shimanami Kaido bike-way, a series a bridges that connects the main Honshu island with the Shikoku island, hopping across a series of small islands. Beware that September-October is typhoon season, and if July-August it is sweltering hot. We went early August and it was great, but make sure you bring enough to drink. Also, I recommend there the hostel "Anago".
http://wikitravel.org/en/Shimanami_Kaido_BikewayIn the Nagano area, Matsumoto is a very nice small city.
Since you want to go to Hokkaido, I highly recommend going by ferry: though it is somewhat slow (generally takes up the whole day), it is a really nice experience (as well as quite cheap): we had a sento (public bath) onboard, and you could relax in the hot-tub while watching the coast gently go rolling by. Also you will usually arrive at a small port town (we arrived at Otaru, leaving from Niigata) at the crack of dawn, the perfect time to go to the local fish market and get breakfast of the freshet sashimi possible!
