I tend to empty my paper inbox once per day (first thing in the morning). Typically, I'm the only one adding things to my inbox, being self-employed, so I don't worry so much about things sneaking in there when I'm not looking. My e-mail box gets processed about 2-3 times a day, and my voicemail usually gets processed as soon as I notice that I have a message waiting indicator.
I think the key here, at least for me, is to process exactly as often as it takes to keep the quantity of unprocessed stuff at a manageable level. I have a friend who's a paralegal in the military, and I doubt processing her inbox only once per day would be adequate. On the other hand, I know people whose work entails large stretches of focused work with little external input, and so once every other day might be more than they need. One thing I've learned to do is to pay attention to my internal "worry meter": When I start to feel anxious about what's accumulating in my inbox, it's time to process it.