Tenon Depth. What's your rule of thumb

[Cliff Rohrabacher]

I just got done playing with my new mortise pal and realized that my bit is to short. It has a 1" cutter, but you lose 1/2" due to the height of the MP.
Time to order another bit

Got to Whiteside and order a 3/8" staggertooth bit they are plenty long.
Either that or get some half inch shank slot mortising bits.

[Jesse Cloud]

Longer is better as long as you can keep the cut smooth and true.

However, all machines have some runout, and few setups are perfectly square, so a little nit won't matter in a 1 inch tenon, but will totally screw up a 3 incher.

Other than leaving 1/4 inch or so left in the workpiece, I cut as deep as the method I am using will do accurately (and funny, as I grow older its not as deep as it used to be, maybe the nit is in the operator.)