I know that tying the knot right before you use it is best for knot strength but how long will you let a knot sit before it's necessary to retie it? At the moment, I am using mono if it makes any difference.

I usually have all my stuff already tied on and leave it on for a couple of weeks, I'll usually retie after that or after I've caught a few fish that put up a good fight. Haven't had any trouble from it.

I only retie when the knot or line is damaged.

Ive had knots tied for months with no problem. The only way I will retie is if the line is damaged. I have never had a knot break on me. My knots are always tight and perfect, if not then I will tied it again before I cast it.

Out of habit, i retie everything the night before fishing. This includes lures AND new leaders. I doubt it has much difference except if you've had something lying around so much that the line starts to break down.

I have a slab tied on right now that's literally been on for over a year and no tellin how many white bass. It's kinda like a lucky charm at this point.