all day, scrubbing a single gun?
Huh?
Or do you mean cleaning a lot of guns, for the whole company?
Depends, but usually just one gun: yours. You almost never clean someone else’s weapon for them. If you are part of a crew-served weapon team such as a mortar, you maintain that system and then your personally issued weapon which is a rifle or pistol.
It can take much of the day to clean one rifle, especially if it has been in the jungle for a week with live fire exercises interspersed. Every rifle I had issued to me was absent almost all of the bluing on the steel parts, so even a little dew or hand sweat would start rust right away.
Even if you finish your weapon quickly, there are always an M249 SAW around to help out with.
The best secret for rifle cleaning - dental pick! Just don’t get caught.
I’m surprised noone has ripped me for using the word ‘gun’ in my original post insated of ‘weapon’ or ‘rifle’ 
Everyone knows that one is for fighting and one is for fun.
Sailors at sea still stand watch on weekends, of course. (You can’t just let the vessel drift all weekend–someone needs to drive. ;))
However, you usually don’t have training, meetings, or drills on weekends. This makes a big difference to a sailor. In other words, if you’re not on watch, and you’re caught up on your personal qualifications, you can do whatever you want, like watch a movie or read a book. More commonly, though, it simply involved catching up on sleep.
Standing watch takes a lot of time, though. Most sailors were on a “1 in 3” watch rotation, which meant that you stood watch one-third of the time–i.e. 6 hours on, 12 hours off. So during a 48-hour weekend, a sailor would be on watch for 16 hours of it, and this was considered a “break” compared to the work week. This is why most of my time at sea that I can recall involved being in a perpetual state of exhaustion.
Chow hall is open 24 hours? Why?
People are “on watch” 24/7. Not the same people the whole 24 hours, but a significant proportion of personnel work nights and are asleep during the day. They deserve hot meals. When I was on active duty a special pass was required to get served at the mess hall in the middle of the night.