I think it depends on what you consider to be "understanding" a horse.
After two to three rides, depending on the type of ride, I can get an idea of what a horse is like. I can have a better approach on how to ride that horse in a way that makes it a positive experience for both of us (usually at least... Lord knows I've met more than a few exceptions to the rule!).
However, true understanding to me takes months of riding that horse in a variety of situations, learning the horse's personality, letting the horse learn about you, etc. But once you have that connection... wow. Amazing.
Of course, then there's the issue of whether anyone else is riding that horse. Because I've ridden more than a few horses that were being ridden by others, and a few months later they were totally different horses.. sometimes good, sometimes bad.