It really depends on the horse and what he wants to do. Some ottbs catch on quickly and find it fun, some of them hate jumping and are better off doing something western and easy. My ottb was very good and willing to take on hunters. I started training him to jump and I got him to going around in 3' courses in 5-8 months. It all depends on the horse, but surprisingly, half the olimpic horses are actually thoroughbreds
I have a thoroughbred mare. She trained to race and then was to short to race. A lady bought her and turned her into a 3'6 jumper. When I bought her I wanted a good project horse, it took a good 2 -3 years.