I have quite literally (I finished up about 10 minutes ago) just done some mods to my Flipper Zero. Throughout the process, I stuck the tiny little screws I was using to the side of my hand for convenience. They were probably on there for ~20 minutes and I didn’t even really notice them.
I’ve left paper clips on there for 30+ minutes without ill effect. Can’t speak to anything heavier though.
The main thing is that the flesh isn’t being pinched enough to inhibit blood flow. Tiny screws and parts won’t be an issue, a decent bolt might be, and any other magnets almost certainly will be.