I was told by the person at the bird shop back in Wisconsin that you should change the nectar at least once a week, maybe more often if the weather is extremely hot and humid. The reason is that the sugar solution breeds bacteria and mold, which is bad for the birds. But far from dying, they will probably simply avoid your feeder - it's probable that nectar turned bad tastes and smells funny to them. After all, we can detect bad food, and I'm sure they have very sensitive taste/smell.
What struck me was that you should probably buy the smallest possible feeder, because what's the point of filling up a big jar if you're going to have to throw it all away every week? This really burned me - I originally bought one with a good sized jar, and now I can never fill it, there's just no point.
I changed it once a week last summer in Wisconsin, and at times it was getting pretty hot up there - around 100F. The hummingbirds seemed very, very happy and kept coming back to the feeder on a regular basis.
Remember to clean the feeder every time you change the nectar. You can buy little brushes made for running through the holes they feed from, that's necessary because those points are probably where the mold/bacteria might be a problem - where the sugar and air come into contact. I didn't use soap, but simply rinsed out well with hot water, and everything seemed fine. I would wash properly with soap perhaps at the end of the season, but that's just a gut estimate.
