It depends on where you live, the weather and where the feeders are located in your yard. Hummers will drink from a dirty feeder with spoiled solution regardless of what some people say. I've seen it happen, and I heard a woman address this issue at a program one day. She said that the spoiled solution can cause a chronic respiratory infection in the hummers.

Anyway, according to website: http://www2.scholastic.com/browse/article.jsp?id=4577it is suggested that:

To curb the growth of harmful molds and bacteria, make it a habit to clean your feeder every three days, even when temperatures are cool. It's much easier to fill a feeder that has not become black with mold! Discard the old solution, then rinse the feeder well with very hot water, using a bottle brush to scrub any hard-to-reach places. Do not use soap or detergent. Or, clean your feeder by filling it with a vinegar solution and some uncooked rice grains and shaking vigorously.

And it is easy to clean a feeder in a 1:10 bleach:water solution. Just soak till clean and rinse, rinse, rinse the feeders and hang to dry. Then fill with fresh solution.

However, if you are in a location that isn't all the hot, humid and sunny, you might be able to get by for a week at best.

Maybe someone out there can give you better information.

I'm experiencing that same scenario right now; on vacation & couldn't burden anyone with the task of changing the feeders. And we've had quite a bit of rain while we're away. At least they have plenty of other nectar sources in my garden. My first task upon returning home is changing the feeders, even before unloading and saying hi to my kitties...

I only let mine go a week or so cause if its in there that long then something is definately wrong! It doesn't last long with my birds!

Here is a link that might be useful: hummingbird food

LT, I don't know if you've had this problem, but this year no matter how clean my feeders are the sugar water will begin to get cloudy in two to three days. And in three days I sometimes see mold. The only way I have been able to stop it is to soak my feeders in 10% bleach, hot water, and some dish wash soap for ten to fifteen minutes, then rinse with hot water over and over.
It seems different this year.