I think one should do Jiuzhaigou 九寨溝 in 2 days at least, if you have time you should do a 3 day tour.
If you don't like photography or nature, you can probably do it in one day, getting on and off the bus at the scenic spots, take a look and leave on the next bus.
However, if you enjoy taking a good walk (not really strenuous, the level is like those Family Walks in Hong Kong country parks), and taking pictures, two days' of time is a minimum for you to enjoy Jiuzhaigou.
There are basically three valleys (溝) in Jiuzhaigou- Rize Valley 日則溝, Zechawa Valley 則查洼溝, Shuzheng Valley 樹正溝. They formed a Y shape.
The best valley in terms of scenery is Rize Valley 日則溝. 1 day should be allocated to Rize Valley and another day to the other 2 valleys.
Basically once you get into Jiuzhaigou, the bus travel onwards up to the highest point on either side of the Y, and then you can travel downwards with bus which stop at different scenic spots.
You won't know which way the bus is going until it gets to the middle- Nuorilang 諾日朗, because the bus driver will be told by the director which way to go only when they get there, if you want to go to the other side, you need to get off the bus and wait for the next bus.
I did Zechawa Valley 則查洼溝, Shuzheng Valley 樹正溝 on the first day because the bus I got on happened to go that way.
I started early the first day, got to the entrance at about 8:30am, but it was so crowded with lots of tour groups, but I finished early that day. On the second day, I started at 9:30am, with a lot less people around, but finished late at 6pm, because there is so much to see in Rize Valley 日則溝.
This is a picture I took at the entrance on the second day. Almost no people!
I didn't take any pics on the crowds on the first day simply because all my energy was used to get on the bus, here's a photo at the last stop of the bus, imagine these crowds times 10 for the crowds at the entrance!
Either get there VERY early like 7am (this is what some travel books recommend), or a little bit later to avoid those doing 1 day tour.
If I were to plan it again, I'll probably start later on the day I'm doing Zechawa Valley 則查洼溝 and Shuzheng Valley 樹正溝 to avoid the crowds, or you can do it on the second day if you think you might want to revisit some of the more beautiful spots after going through the two valleys.
Some people do it in 3 days, one valley each day, I think if you do it in 3 days, you can walk a lot more, which is good. Because the bus does NOT stop at EVERY scenic spots, so in order to visit all, you need to walk. Due to our limited time, we chose to walk if the bus stops were nearby (within 1-2km) or if the spots could not be reached by bus but are really beautiful according to travel books.
You are going to miss a lot of things if you just get on and off the buses, that is why I recommend TWO days minimum.
One very important thing- when you get on the bus, sit at the FRONT! otherwise you won't be able to hear whatever the tour guide on the bus are saying, because those people in the bus will be talking VERY LOUDLY!
I can't hear a thing on my first day. The tour guide tells you important information like where the next bus stop is, what you can walk to from a certain bus stop. Therefore, sit in the front! (Unfortunately only Putonghua tour guide is available, I am not sure if the guides can speak English, but good English guidebooks can be bought, and lots of people who don't speak Putonghua are doing the trip on their own, so it should be ok.)
