Measured one i had in my parts bin, still new in the bag. I didn't measure the full length of the screw since the head itself is of no consequence. I measured from the base of the head to the tip. (the shaft in other words) I got 0.370" I have one in my parts bin that was shortened, severely, and it measures .315" (wow). I also checked Jerry Kuhnhausen's Shop Manual, and all he says is to use on the proper length. There is a procedure for using a hanging weight to get to what he considers the correct tension. However, for most if us, we'll just lighten it till we still get reliable ignition and call it quits.
Caution, YMMV and I'm betting that minor variations in frame make a big difference here, so don't use my measurement as a reference. It's just what came from the parts house. It may be that they are shipped too long expecting you to file it down to whatever your revolver needs.