Well, one way to do it is through. All the way to the other side. If you don't want to see the end of the tenon, then going about 2/3 of the way is fine, but basically, I think you can make it any size you want.
On M&T apron to leg joinery, I usually go a little shy of when the two tenons meet, and end them with a miter. On doors, it's pretty much whatever you want.