I think a fail safe defence is 4 moves away.
1.h4 2.Rh3 3.Rd3 4.Nf3 and then simply repeat the moves nc3 then nb1 continually.
This stops the enemy knights getting a smothered mate on you by blocking the spaces d3 and f3 while your king is surronded by his own pieces.
Even the method of block the squares c3 and a3 to stop the white knight from repeatably doing the same moves doesn't seem to work as the knight has a lot of empty squares around.
I think the game is drawn by best moves, but perhaps somebody can find a way to stop any such defence quicky enough?

In this diagram you could never checkmate because the king can just move back and forth.