oops I did not read the question just did the poll i would say 1.5- 2" but to be honest I never had them in all these years of reefing. but from what I see on here that seems to be the sweet spot.
I am in the same spot right now. Just added 40lb of live sand into a 20long and have a 2.5in sand bed. Wondering if this is too deep... Willing to siphon out some into the sump to add sand there, but initially wanted bare bottom in the sump to siphon detritus. Following on this thread for goby/shrimp sand bed suggestions