To LEARN? AKA if you were going to a dressage university and had all of the horses provided for you? I can only give you a very rough estimate...typically many years - anywhere from 10 to 20 or more depending on all kinds of factors, such as your body condition and preparation to ride upper levels. For many of us, it's a journey more than it is a plan. And in that journey, you have to continually REACH for your goal and keep it in sight, while figuring out how to negotiate the realities of life. Youtube Natasha Althoff - she talks a lot about becoming a Grand Prix dressage rider, and the original plan she had to do it in 10 years. Quite charismatic and positive.
No diet, no hoof. No hoof, no horse. No horse is not an option!