I would say 5-6 inches of a fine sand is perfect in my experice.Some people go 6-10 or more.I have seen.I would say 6 is a good number cause remember you will lose some sand over time.
I've also heard people recommend keeping your DSB in a separate fuge type container connected to your sump. So when it gets old you can replace it without fear of crashing your tank.
There are a few good articles in regards to DSBs....the depth depends on grain size. The smaller the size the shallower the bed for it to be effective.
i can't remember exactly but in an old article on DSB and their functions, it noted that they are almost useless unless properly set up through enough depth / surface area...which is why most ppl prefer a remote DSB instead of in their display. i *think* it was 6''+, someone feel free to pull up the article, my memory's drawing a blank right now...
Also depends on what type of animals your planning on keeping. Certain wrasses, Jawfish and inverts usually need a deeper bed to adjust to captivity well