It depends on the horse. It can take months or even years depending on your body, before you're able to handle the kind of intensity and volume a horse brings to the table. But I think the biggest complication is getting a suitable horse. Preparing yourself physically is just one side of it. Preparing the stallion mentally is another hurdle, that is for most far bigger than the first. If the horse has no experience around humans on their own two feet, like the fact that he can knock you down by simply walking into you very slowly, you may find him not thinking twice about wielding his weight with caution or care. Getting him to understand that he can kill you far more easily than another horse is one step, the step after that is making sure he wants to avoid doing so.
Also be aware that vaginal and anal are two very different beasts when it comes to intercourse with a stallion. I only have the experience of myself and a few close friends to go by, but essentially, anal is the more accessible option to doing it right, while also being able to kill you no matter how experienced you are. Meanwhile for vaginal, you're generally unable to take enough of his length to safely and properly have intercourse with him, and training yourself for more depth vaginally is not an exact science by any means as your physique sets a lot of the boundaries. Some people can manage a stallion that isn't mini, many more can't. And no one should attempt to handle something that is getting close to a meter.
If you do want to pursue this, you'll need a lot of money, a lot of time to put into it without getting any sex from it, and accept the risk of a lot of trouble.
