Not sure you will care for my comments. Do not confuse my statements here that I am saying a DIY rebuild does not function ( I am not saying that).
On the flip side: transmission parts failures , screw-ups, mistakes from the lack of knowledge are expensive
Also remember There is a difference between replacing parts and rebuilding something properly.
A 4L60e is not a DIY friendly trans. Too many thing can go wrong , not checked or over looked by a DIY.
I put the numbers at '10' on those questions.
Hours needed for rebuild? I know how long it takes me. Can not comment on others.
Success rate for DIY - backyard mechanic? Unkown by me.
Failure causes? I doubt it is parts.
I've chatted with customers that would tell me how the parts we sold him were the problem. Yet I use the same parts in 250 - 300 rebuilds of 4L60 trans every year. So is the failure the cause of our parts or the lack of knowledge and ability?
It has been suggested to me a dozen of times I should write a book on trans rebuilds.. I point out that there are books and videos already available.
Over the years I've had DIY person hand me a rebuild manual as they wheel in boxes of parts from their failed rebuild attempts.
Seasoned mechanics bring in transmissions they built that would not work properly. Upon inspection I find glaring errors.
If you wish to attempt a rebuild ... go for it. I answer questions when needed. You may have "that ability" and all works out well.
Over the years I have watched some folks develop into excellent trans techs. There are a couple on this board
Dont worry how old you are... it is the mileage on your body that adds up.