Three moves, and one of them was a draw
Round six, move 28. He gave up his rook on f7 to stop the mate and there were three ways to take it. I thought about two of them, Qxf7 and Rxf7, and I played Rxf7 because it forces him to give a check I can block and then trade into an endgame that is completely winning.
The engine puts Rxf7 at +7.99 and Qxf7 at +4.01, so the move I picked is worth about four pawns more than the other one I had in my hand. The third move is 28.Rxh6+, the sacrifice that opens his king, and it is the one the position is shouting for. It is 0.00. Dead level. I never looked at it.
One more thing the engine says that I did not know: after Rxf7, every black move except Qe1+ is mate in one, and my reply Rf1 is my only legal move. So the endgame was not a choice he made. In round five I had the same kind of decision on move 29, took the complicated one instead of the recapture, and lost a rook. This is the same decision with the answer named out loud first.