 Nectar in the feeder itself should be tossed after two to four days, and that's really dependent on how warm the weather is. The feeder should be clean and refilled with a fresh batch of food. If there's any doubt as to whether that solution has spoiled, you want to err on the side of caution and just discard it. Just as eating spoiled food can make you sick, the same is very true for hummingbirds. And imagine a hummingbird puking at 50 miles an hour. That seems terrible.