I'm not quite sure what you are asking. Is it how far to thread the valve onto the through hull
If it's tapered, you can see it, just hold it up in front of you. If you can't see it, take your handy vernier calipers, and see if the threads are bigger at one end. If it is tapered, that governs the depth
when you screw it in.
What material? Bronze
, or plastic?
How big (diamter)? 3/4 inch is fine for a 3/4 inch connection, but plastic at 1 1/2" I'd want some more.
Usual standard for bolts into nuts is 2 1/2 threads past the nut.
The 50# side load ABYC standard applies - You should be able to apply a load of 50 pounds at 90 degrees from the axis of the fitting, and not have it break. Shove it with your foot.
This is a shotgun answer. If I missed your situation, tell us some more.