Generally 6-8".
To be honest ive never seen any deffinate advantages with going over 6" deep. Imo, surface area for diffusion is better than extra depth. but it needs to be of a decent size to make the bed worth while. Too small an area and you end up with rapidly diminishing diversity as one life form outcompetes the other...the bigger the bed, the longer this takes to happen.
I generally run the rule that anything smaller than 24" x 12" isnt realy worth it outside the main display. It wont harm the system but the rewards will be greatly reduced imo in terms of nutrient control and live food output.
There are loads of factors to consider and I could ramble on all day about it, suffice to say, here's my take on the theme which has served me and others well over a great many years and a whole host of systems.
http://www.reef-eden.net/DSBs.htm
http://www.reef-eden.net/new_dsb.htm
regards
Simon.