Never, three desktop PC's here have run continuously for over a year or so, only being shutdown if there's a problem. Greta eat your heart out.... Several Raspberry Pi's run 24 hours a day, seven days a week,too. Having said that I am looking at a quarterly bill here of over £1100, so there is a downside.
If I remember I hit the power button to put it to sleep. If not, it goes to sleep after 15 minutes anyway.
Convenient because it fires itself up for scheduled tasks/updates etc then goes back to sleep.
Always shut down for the night. During the day it depends, if I know I'm gonna use it again within like 30min then I keep it on otherwise shut down. With the speed of the M2 drives it only takes seconds to boot up.
No, it makes next to no noise and is in use day and night. When idle it throttles down to a few hundred mhz anyway and all but the GPU use a couple of watts of idle power whilst the GPU sits around 18 watts.