Thanks for the question, since that is one subject that is near and dear to me.
Assuming the DSB is set up with the proper grain size (sugar grain sized or smaller), and that it came loaded with the microscopic worms, pods, and all the other multitudes of life forms that make live sand alive, then the issue we face per maintenance only involves those life forms.
As with any ecological system, a sandbed has both predators and prey, and as with any small enclosed space, the predators through their feeding and their own reproduction will sooner or later devastate (eat) most all of the prey, leaving the predators to starve to death, which in turn leaves you with a dead sandbed. And a dead deep sandbed can start to turn really nasty. This is exactly why DSBs got a bad reputation, not because they will always be bad, but simply because the hobbyist sets up a dead DSB or allows it to die and is then surprised when it fouls his tank, and instead of blaming himself for not understanding biology, he blames the DSB.
To avoid the eventual shift in predator and prey populations, I have always removed 1/5th of my sandbed and replaced it with new live sand every two months. I do not siphon out just the surface of the entire sandbed, that would be a bad thing to do (biofilm is on surface). Instead, I section my sandbed into five areas and starting at one side of the tank, I siphon out one 1/5th area from top to bottom and replace it with new live sand, then two months later, I move over one area and do it next and so on and so on. Its not that I want or need the new sand grains, its the life that comes with it.
As you can see, such a routine can become a chore as well as an expense for those that must buy their live sand, it is also a waste of sand as well. These are exactly the reasons for why I collected those sandbed synapids and sea cucumbers. Ever since I add three of those synaptids a year ago, I have not changed out any sand. Instead of having or worrying about predator/prey populations, I can sit back and let the synaptids take over the job that the multitudes of microscopic life forms once performed. You will always have some microscopic life, but most likely not in the amounts needed to keep a DSB fully functional. Last month I also added a sandbed sea cucumber to my DSB and it is doing very well also. Last night, by chance, I also saw that I still have a few sea biscuits (sand dwelling sea urchins) in my sandbed as well. Please see my live rock/sand page for more details and links. For photos of the synaptids, sea cucumber and sea biscuits, please see the following pages as well. Thanks.
Chuck