Mortise Tenon ?

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.
 
I usually make them about an inch long for apron/leg joints, and I have not had a problem so far. I use a plunge router to cut the mortice. It depends on what the situation is I think. I did a sleigh bed and I made the joints an inch and a half into the end posts, so far so good.
Chuck
 
Depends on what it's doing, size, whether it's pinned. I like to go 3x the thickness of the tenon if I can. Not sure if I read that somewhere or not, but it works pretty well.

I've always wondered on a pinned joint how much wood you need to leave past the pin. The few I've done, I left about 2x the pin diameter, but that was just a guess.
 
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