as often as needed...
I like to "power balance" all of my manual GB Tesoros right to the point where they talk to you just a bit.
If they get too chatty, I check the GB and usually bump it a bit more positive; too quiet and bump it more negative. I like to just hear the occasional chirp that small hot rocks will make. It only takes a couple seconds to reset the GB as you're moving from spot to spot, and it's even quicker and easier to judge by "power balancing" in disc. mode, rather than setting it in all-metal.
Running GB as negative as possible without constantly chirping, seems to give best depth and target response. I like mine right on the edge, and when on the edge, it's easier to tell if too positive or negative.
Too positive, and you'll lose target response on larger coins. Too negative, and the detector sees all the ground minerals as targets and chirps away as you swing.