40 minutes in the sewer level in death toll. We used a glitch that I will describe here.
First, grab a propane tank and look for a crate on a trolley below where you need to push the button to call the horde. Next, using the propane tank, knock the crate off so it is barely leaning off the trolley that it is nested on. Make sure it's falling off in the opposite direction of the button. Next, throw a pipe bomb in the corner between the crate and the trolley so that the crate is blasted up one level and should be next to a box with pipe bombs and molotovs on it. If you jump on the crate you should be able to get up to a space where you can not go in the campaign and only the infected can go in versus. From here you can get all the way back to the safe room at the beginning of the level. In my experience only hunters and common infected are capable of getting to that point, and with only one point of entry they can be easily dispatched. There is also a steady supply of ammo there so really I think you could hold out for as long as you'd like if you can get there.