There are many factors and I don't know all of them but generally speaking if you could isolate each, I'd say it's uniform. If you give legendary magical item to X, then you'd get the rise or fall for Y hours and the veil would go back to Battle normal which is just below Nexus normal, which is slightly less mana in the world than ideal.

Villagers, especially ones with large kill counts shift the veil, so if one were to log in when you give your item, then the shift wouldn't likely last very long if at all. You'd likely need multiple items to swing it.

Conversely, if Scion Mage Leader logs in, you'd expect magic's level to rise and your item's affect may seem longer.

This affect will bleed down with passing game time. As far as staying, I believe it will completely reset on reboot if I recall correctly.

I'm sure there's more to it but that's some of the things I noted when I played Nexus all the time.