Automatic feeders are pretty nice to have in case of emergencies and business trips, but most well-fed, healthy fish can go for a good ten days without food before they start to suffer.
The thing with an automatic feeder is they may add too much food to the tank and you can return to a whole bunch of dead fishies from the ammonia spike all the rotting food caused.
When you are home, feed your fish moderately at least three times a day. Make sure your tank is spotlessly clean in the water quality department. Then when you leave, it is usually best not to feed the fish for that duration of time. My fish have survived foodless during all my three day weekend vacations. I don't go on them very often, about once a month, but it doesn't hurt them to fast so long as they are healthy before hand. They are a variety of tropicals and goldfish.
If you have a planted tank, use a timer to set your lights. For unplanted fish only tanks, keep the lights off when you are gone.
If you are going to invest in an automatic feeder, make sure you get a really good quality one. Nothing could be worst than having a feeder that just dumps all the food into the tank giving you dead fishies on your return.
~~Colesea