If he's got a good track record, right away.
I didn't ride mine right away because we had been working on some issues and so I worked on ground work and various things for a month before I got on. In hand trail rides, lunging, in the round pen, free lunging in a polo arena, etc. So by the time I got on him, his habits had lessened and we didn't have any problems.
So it's up to you :)