user13744: Except for the variations where there is a capture or fork possible for the defending side, is a position possible where this kind of endgame will last more than 50 moves? Is somewhere a calculation in moves done what is the worst possible situation?

user11629: It takes at most 33 moves to win this endgame from any position (excluding positions where bishop or knight can be captured). See https://en.wikipedia.org/wiki/Bishop_and_knight_checkmate and references there.
So, no, a forced draw is not possible in this endgame. However if the defending side is playing perfectly (e.g. an engine with tablebase), it could be very hard to win this for a human within 50 moves if you start from one of those 33-moves-to-mate positions. 

13744: <p>Except for the variations where there is a capture or fork possible for the defending side, is a position possible where this kind of endgame will last more than 50 moves? Is somewhere a calculation in moves done what is the worst possible situation?</p>
 11629: <p>It takes at most 33 moves to win this endgame from any position (excluding positions where bishop or knight can be captured). See <a href="https://en.wikipedia.org/wiki/Bishop_and_knight_checkmate" rel="noreferrer">https://en.wikipedia.org/wiki/Bishop_and_knight_checkmate</a> and references there.</p>

<p>So, no, a forced draw is not possible in this endgame. However if the defending side is playing perfectly (e.g. an engine with tablebase), it could be very hard to win this for a human within 50 moves if you start from one of those 33-moves-to-mate positions. </p>
