Uhh...I don't think there's a union for these things, so you don't need to worry about work rules, overtime and all that stuff. It's a just piece of gear, run it however it meets your needs. Use the drive idle settings to save wear and tear on those pieces if you aren't actually hitting the drives 24/7. The DNS-321 is pretty efficient at idle, I measured mine at 8 watts which is basically two night-lights.
I turn mine off when I know I won't be using it for a couple of days, but that's just to save energy. I'm not remotely worried about wearing anything out, and I keep it where it's easy to get to when I want to use it. If I kept it in a basement, or was running automated overnight backups or anything like that I'd leave it on all the time and not lose a moments sleep over it.