Like others have said, it’s not necessarily just a time thing, more of experience and knowledge (which does usually come with time, but some people have ridden for years and still don’t understand common horse knowledge). You should be prepared to work a lot more than you would when just taking lessons. You should go and groom them or work with them every day, and that can take a few hours each day. I try to spend at least an hour with my horse everyday, whether it’s grooming, riding, lunging, etc.. I’m not sure if your barn would feed your horse or if you would have to, so take that into consideration also. Maybe try leasing first? It will slowly start to add on responsibilities instead of getting them all at once. Good luck!
Depends on your skill level. If you are a beginner I would wait a while till you are more experienced but if you have been doing this for a while and know your stuff I would say go for it. Because you never know what horse could be yours and how it acts.
I don’t think it is really how long type of thing, it is more of a how experienced are you and how dedicated are you to the animal? You may only have ridden for a year, but you may be at the barn all the time learning horsemanship skills. If you know how to take care for a horse, and have been around horses for a decent amount of time, I say try it out with maybe a lease at first with the guidance of an experienced adult in the horse world. I know every little girl would say that they are ready, but when you know how to take care of a horse well, and the fundamentals in the discipline that you want to try, go for it!