When you arrive at the top of your sleeve, it should be whatever length you want from your ARM PIT to your wrist. I'm doing mine a bit long because I like them that way for outside jackets, which is how I will use mine. The proper length is entirely subjective - whatever you like.
HOWEVER. You have to end your sleeve on the same row you ended the body. Let me repeat that. END YOUR SLEEVES ON THE SAME ROW YOU ENDED THE BODY. All three parts have to be on the same row when we join them up. To make it easy, end on the last row of the pattern. You don't HAVE to, but it's easier that way.