For 1/2" NPT, the specified total engagement is .534", consisting of .320" (4½ turns) by hand and .214" ( 3 turns ) by wrench. Then it gets tricky: ANSI in their spec acknowledges that in practical field situations, threads are often cut to a shorter total length than specified. Hence the engagement length and # of turns go out the window. Then, when using teflon tape, the reduced friction may allow hand tightening beyond the "spec". In steel pipe, overtightening is usually not harmful. With plastic pipe, overtightening usually destroys the threads and causes leaks. Plastic pipe manufacturers specify 1/2 to 1 turn MAX beyond hand tight. In your case, wrench tightening beyond limits, the "weak link" was the copper tube itself.
This is one of those cases where a "feel" for the job is developed with experience.