A bag of sugar will keep for years. When it is mixed with water, it can grow mold. The water around here has so much calcium, that if I boil the water, then dissolve the sugar in it, the calcium comes "out of solution" in a day or so and turns the water cloudy. This makes the nectar look "bad" way before it goes bad. So I don't boil the water, just mix the sugar with hot water. Works fine, and doesn't cloud up.
You can soak the feeder in bleach water, then use an old toothbrush to scrub the flowers and other parts that attract mold. I am finding a lot of feeders can't be taken apart, to clean thoroughly. Grr! I think Perky Pet brand is one that allows you to take them apart for cleaning. And some replacement parts are offered.