I think it depends, do you have the experience, the confidence, will you be dedicated, can you afford it, Talk to your trainer and you could also give leasing a try.
I rode for 8 years before I got my horse, and my parents made me wait until I was 16 so I could get a car and get to the barn everyday when needed.
You should lease a horse first. This will give a feeling of all the work that goes into owning a horse. This will also give you the chance to spend time with the horse, practice, but you are only spending money on the horse part, not the vet, the ferrier, etc.
I would say 1 1/2-2 years as long as you have a confident rider and forgiving horse.
Its Aldo depends on the confidens that you have riding
It truly depends, maybe ask your trainer I think rather than how long it should be based off skill and experience (which does take a while). Maybe also give a go at leasing a horse.