It varies - from 12-15 cm to 5 cm.... I had some "sand storms" in the tank, the currents in the tank rearranged the depth of the sandbed.
I'm using river sand (i've washed it a couple of times before adding it to the tank)
My sump is the same dimentions as my display - with a DSB as well - the depth of that DSB is at min 16 cm...
I've added this sand bed for a couple of reasons:
1. the crushed coral that i've used, collected alot of detruis - the sand bed is better in that regard.
2. I can see alot more small critters in my tank since i've added the sand - they just love it! alot of codi pods, bristle worms, bristle star ext...
My nitrates other levels are almost 0 since i've made this change.