Re: How long does it really take?
If you pretend for a minute that there is an agreement on what "useful" and "combat scenario" are, you can imagine a scale that measures the time you spent training vs. how "useful" this training is for a specific teacher/club/dojo. For certain teacher/club/dojo your "useful" value will be pretty much zero across the scale, even after decades of training. For others there will be slow or fast increase as your practice continues.
Use common sense to avoid the zero value case.