How many moves do I have to checkmate a king with just a knight and bishop?

Let's say I have a Bishop and Knight vrs a lone king. I know that I have a limited number of moves to checkmate the king otherwise a "draw" is declared. I heard this from tournament rules. How many moves do I have to checkmate the king with a knight and bishop?

its 50 moves, it can be done in about 35-40 moves

That depends on where the king is.

After 50 full moves with no capture or pawn move, either side may claim a draw.

You would have 50 moves to checkmate--if you don't checkmate within fifty moves (for both sides), the game will be drawn due to the fifty-move rule. If a pawn move is made (which it wont be if you only have a knight and bishop ), the 50 moves start over. Also, a (black) capture would result in a draw (by insufficient material), since you cannot capture Black's king.

You should be able to pull it off in 25-35 moves if you master Philidor's standard W method and practice the mate alot against a strong club player or Master willing to play the role of defender.

Tarrasch teaches it at the very beginning of his book he insists that everyone should know it which is contrary to modern Masters who say it will rarely appear so it's not worth studying and doing the hard work to learn it.