Anything by carthief takes ages to clean
Dunno a few of mine might take 3 or more hours
It might not say how long but usually says how big the map is
Personally I don't like a level that takes more than 3 hours or contains too much double handling or too much walking simulator (I always add a way to cut corners in my levels)
Underground research as an example when it comes to the incinerator although the map looks pretty cool
Carthief is also working on a new project that sounds like a cleanup adventure with 4 levels tied into one
Canonfodder has a few big maps
So do other creators
Sub n try em out

Some of my maps might be tiny but can really get messy depending on how the janitor cleans it
Garbage chute can be done in half n hour if done the right way or 6 hours if it was cleaned the wrong way:)

4+ hr maps take a fair bit of work, most creators hit on avg 2-3 hours. That said there are some big ones on there, you'll just have to sift though them and check if they say how long.
For mine, maybe Gone postal and St Bleedout hospital. They're maybe 3-4 hours ish