I started metal detecting this year and FAR from an expert. My rig is a minelab 800 if that means anything to anyone, if not its one of the better rigs on the market.
My point is this, after digging beaches, parking areas and hiking trails I've learned you always find something where you find people. Not much on hiking trails but more around areas with a place to sit or fish. If you find something you will find more stuff as long as you keep looking. Sometimes good stuff is found under or near trash finds so its worth the effort keep searching.
If a lot of my finds are trash found deep or in hard ground I'll spend less time in that area.
If I'm finding pennies or any coinage, I keep digging. When I find trash I keep digging but learn the signals in that location and quickly move on.
What I'm trying to say is if I don't get a signal I don't dig.
From what I understand some of the better rigs can find coins a foot or so deep. I've found a 2# barbell weight about 18" down. Size matters, the wider the target the easier it is to find when deep.
Maybe more experienced diggers can chime in