I've always heard 1" threaded in to a weld in bung before the jam nut. Now with my lowers I pushed the front a little bit more forward and I know I don't have 1" in the bung. It'll work until I can build new lowers later on.
1/2 the length of the threaded stud should be a good rule of thumb. What joint only has 1/2" of threads?!