What is the farthest away a game can be from a guaranteed checkmate? What is the maximum number of moves away from checkmate that a player can be where checkmate is guaranteed? There are many positions that are a guaranteed win for a player if played perfectly, right? (Obviously there is not a guaranteed win from the first move). My question is: What is the furthest amount of moves one can be from guaranteed checkmate, when the shortest amount of moves to checkmate is taken?
Let's assume you are white and have, say, a bishop and a knight left and black is down to just the king. (there is a way to get checkmate with a bishop a knight, so checkmate is assumed to be guaranteed). Now, one set of moves could take say 40 moves to get checkmate from this point, but the least number is say 19 moves away. Now, my question is asking what the most amount of moves to guaranteed checkmate is, but you have to take the route to checkmate that has the least amount of moves, so 19 would be the case from the point where the bishop and knight are the only pieces other than the kings left.
I apologize for the lengthiness of this question, I couldn’t think of a good way to ask what I’m asking. This question derived from wondering if black or white could win every time from the first move but obviously that isn’t the case. The main question is at the end of my first paragraph.
Edit: 50 moves without a pawn push or a capture does not trigger a draw.
I doubt that anyone will be able to give you a definitive answer beyond this one, but the longest known win from the end of a game is a famous 549-move 7-piece tablebase win. Until we get to 8-piece tablebases, this is probably the longest anyone can prove.

You can see that position, and all 549 moves here.