I have seen them clean a train in Rutland VT and it takes 4-6 hours
The train in Rutland has 5 cars and has run 7 hours or so up from Rutland. It's cleaned by the Vermont Rail System.
I'd assume a longer train would take longer to clean except that you could increase staffing levels and finish the job quicker. Perhaps it makes a difference if it's Amtrak staff or a contractor as in this case?
Back when I was a conductor I learned I could make a big difference for the coach cleaners if I came through the train before large stops with a trash bin for passengers to throw things away themselves. It made a big difference, but few conductors do that.
Christopher