Many many years. You cannot train a horse if you don't have a good solid base of knowledge yourself. Horses do not do everything by the book, and you should have gained plenty of experience of all the surprises that they can throw at you before you consider yourself capable of educating one from scratch. There are a lot of badly behaved, badly trained horses out there - mostly because their early years have been managed by someone who thinks they know more than they actually do.
Depends how much support you have as well. I backed my first horse a few years ago, but had the help of an incredibly experienced friend, who was on hand every day. I knew pretty much what I was doing, but she was there to fill in the blanks when I had a blank moment. I also found it useful having someone on the ground that could clarify that what felt good looked good as well.
Depends what you class as experience. I have trained both my ponies to drive from scratch (both didn't lunge or long line, so literally from scratch) and i have trained one to walk who was never halter trained (was 4yo) but they are mini and generally easier to train than a horse. I have been riding since age 2 owner since age 12, but i had never trained a horse to do anything before the minis.
Depends completely on the horse and how confident you are.
I got my first youngster (who spent more time in the air than he did on ground) when I was sixteen, and went on to sell him to a child a few years later. She took him to PC and all sorts!
(Should probably add that I had been riding for nine years most days at that point, though!).
I would say that I am an "experienced rider." I have hunted and evented and show jumped, but I have always been a "weekend rider" and have never worked in the industry. I trained my current horse from a 2 year old, and he was extremley well handled when I got him from the stud and was used to having tack on, feet, grooming, lunged and he was a sweetie and never caused me any problems. But I made lots and lots of mistakes, and how he is now is all my fault! Not that he is bad, he just could be better and probably has never reached anywhere near his true potential because of what I did or didn't do, through lack of experience of bringing on young horses.