My method is to start checking about 3-4 weeks before travel and check regularly up until a week or so before travel. I usually have a particular time of day I prefer to travel so I tend to prefer not to wait until the day or two before travel. I have found that sometimes a steep discount (70-80% off) will pop up 3-4 weeks out and if it does for the day and roughly the time I want I will snag it. Generally though the standard discount is 30-40% off, sometime 50%. Then often in the last week before travel sometimes a good discount will appear. Since you have two days to play with and seem fairly flexible on time then you should be able to hold out for a good price. But I would recommend a periodic check early on just in case a nice discount appears in which case I would go for it.

Also another point that you may or may not be aware of, Shanghai has two airports one is PVG - Pudong (handles International and some domestic) and then the old one is SHA - Hongqiao (handles most domestic). There is quite a distance between the two, some of the others here can probably give you more specifics. There are some flights from Pudong to Kunming but only a few a day (3 a day right now) most flights are from Hongqiao (right now 12 a day). This should be a factor in your planning, just be aware as this may affect your hotel choice and other plans.