Well it really all depends on what type of fish you keep. Some can be fed flake food, while others really should have frozen or live food. Other fish like tangs, need algae in their diet. Your best bet is to research your fish types and find out what is best for them.
As far as feeding frequency goes, I feed once a day with high quality flake food or frozen mysis/ chopped seafood. I only put in enough that it is all consumed within a few minutes to help keep unnecessary waste out of my system.
When you go on vacation, you can use an auto feeder. I am actually using one now for the holidays. The only thing is that I have only ever seen auto feeders that work with dry food, so your fish have to accept flakes or similar food. My auto feeder has 4 times that I can set if I want, plus it allows me to choose how much food I drop in the tank through an adjustable sliding door and the # of rotations that it will perform at that time.