if anything, turning off the phone during those 8 hours would be worse, as starting back up takes a surprisingly high amount of battery. but inbetween shifts it'll likely help to turn off the phone as long as you are sure you won't need the phone at all during that time

as one of the other comments mentioned, enable airplane mode to save a good bit of battery. while there is an app meant to help in simmilar scenarios, ForceDoze, it's annoying to set up and requires some quite nerdy stuff.

Airplane mode + extreme battery saver mode + delete all the apps you can

And check your battery stats to see if anything else is using power

I'd keep it on during the shift, and turn it off outside of it. If you don't expect calls or other communications during the shift, put it in airplane mode for maximum savings

To answer the question in the title - it varies a lot. For example one of the things I hate about my (Exynos) S22 Ultra is the high "sleep mode" drain - about 1.5% / hour on Wifi.

At the same time some other phones I have drain like 3% in 24h under the same conditions.

I have yet to find a good pattern what causes good standby times vs bad ones but in general Qualcomm chips perform better than Mediatek Chips which perform better than Exynos chips. And generally speaking ROMs that are close to Stock Android perform better than heavily altered ones.

For optimizations - best thing you can do is turn off Location Services and Bluetooth while you don't need them. And Airplane mode also helps.

Also I think since the phone "wastes" a lot of energy during bootup its probably better to just put your phone in airplane mode instead of turning it off.

Turn it off when you get off work.

I think it depends on the device and how far you've gone to prevent energy use on it while it's idle. It sounds like you probably don't have any third-party apps installed, so that's a good start, but removing anything else you don't use would be good, too. Or at least disabling them as much as possible.

Depending on the actual usage, which will vary from device to device and especially OS to OS, I'd either choose to turn it off after work, or off after each time I'm done using it for an hour or more.

The other factors are how hard it is to charge, and how long it takes to boot when you need to use it. If it's easy to charge, I'd probably not worry about it much. If it's slow to boot, I'd lean towards not shutting it off during work.

Pixel 8 & Grapheneos, about 4-5% over night and two app still working Battery Botpro and Rethink. Not use flight mode or battery saver.

Why not just get a digital camera and a torch?