Nominal for most of the Dan Wessons was 0.006. For the long frame SuperMags it was 0.003. Since the gap is adjustable, I've run a series of tests over the chrono with a 22 DW and a 44 Mag DW. When the gap gets to 0.012 the revolver will begin to spit crap back in your face.
A couple of considerations. The cylinder face is not a perfect plane at a perfect normal angle to the axis. The cylinder gap should be measured at the chamber where it is tightest if there is a detectable difference from chamber to chamber.
For serious work where reliability is the primary concern the gap should be in the 0.005 to 0.008 range with 0.006 being optimum.
For plinking, small game or deer hunting where reliability is not life or death it can be a bit tighter. Taking it down to 0.003 presumes a premium grade revolver that has a dead true cylinder face and minimal end play.
I think S&W allowing it to go to 0.010 stinks. And going to 0.012 is beyond the pale when you get crap blown back in your face.
Surprisingly, the cylinder gap does not have as much effect on velocity as you might suppose.