Actually it depends on how tall you are, how high the keyboard sits on the stand and how long your arms are.
Ideally it depends on your proper keyboard posture. Your forearms should rest comfortably at a slight downward slope facing the keyboard. Your wrists should be comfortably at a level where they bend slightly downwards onto the keys.
This is why a proper piano stool should be adjustable in height.
Your elbows should be about 5 to 8 inches above the keybed.
It can get tricky if the key bed is thick.
If you the adjust the keyboard downwards to allow for your wrists to be above the keys, then the bottom of the keybed can squash your knees.
No room for legs.
Its a fine line between pleasure and pain. Ray