It doesn't matter. If you k ow how to take care of a horse and understand that they are not yours then you should be good. I don't mean to sound mean but you must take care of the loan horse well. My current horse was a loaner and he was in bad shape. He had no muscle and was skinny.
I honestly think that in order to have your own horse you must know how to tack up, untack, clean stalls, and groom the horses by yourself and you must feel comfortable riding as well by yourself
I honestly doesn't think that it matters how long, I think it matters that you can properly care for the horse, and are going to train/care properly. Remember that the horse is under your care.
I think a year is good just 4 getting more experience
I wouldn't say how long , I would say how good you are is a very important thing when you loan a horse.
Are you able to control a horse in any situation ?
When the horse gets scared , bucks or starts running ?
Do you have professionals around you that can help you with the horse when you have questions or when something is going wrong?