There doesn't seem to be any discernible way to tell how long a night lasts for.
At one point, a night ended as soon as the sanity meter went to 0. The night after that, it kept on going and going... I got caught, and very shortly after restarting that night, it ended.
Edit: Almost immediately after I posted this, I just had two of the same bunny girl stacked on top of each other on the left window. One eventually disappeared. Somebody else appeared in the right window. I had both doors closed, and had a fair amount of sanity left for the time being. I then got jumpscared... by nobody.