I just did Nootka last summer, it was amazing...I've heard from a number of info sources (and hikers we met along the trail) and August/Sept is often viewed as the best time to hike the trail, since there is a better chance you'll have good weather...plus, the later in Aug you so, the more ripe the blackberry bushes will be when you reach Yuquot (there is nothing like feasting on blackberries after 5 days of hiking). On the otherhand, we heard from friends who hiked the week before us that the local Nation had a huge festival, and they were invited to participate and feast on salmon and other traditional recipes and goodies...might also be worth thinking about.
We took five nights on the trail, which was more than enough, you could easily do it it four, but some groups prefer to spend two nights at Calvin Falls. Remember that if you're timing your trip with the aim of taking the MV Uchuck ferry out, the trail will be much busier, since that will be what others are planning as well. We had a good sized group, and it ended up being almost the same price to fly both in and out with Air Nootka, as it would have been to fly in and take the Uchuck out, plus it was much much faster. Goodluck!!
ps. if you are interested in booking the cabins at Jewitt Lake - you should book well in advance, since it's my understanding from speaking with the locals last summer that they book up early!