What is the average of the ones you have dug?
Mack-digger, there is no "average". About the only "average" you will find, is site-per-site "norms", for *just* that site. And even THEN flukes will exist.
Turf hunting (parks, yards, schools, etc...) in un-disturbed turf is where there is usually stratification. And each location has its own set of norms, depending on soil type, moisture, how they mow the lawns (whether the mower picks up, or just leaves the mowed grass back down, etc...). So for example at some park turf, the '40s/'50s wheaties and silver seem to start at 5". And to get to the '20s/30s silver, they might start at 7". And then the barbers and IH's are down at 8" or more, and so forth, and so on. At other parks I've been at, sometimes there's still annoying '60s memorials down to 7 and 8"

And other parks it's all reversed, where the wheaties and silver start at a mere 3".
But other types terrain can be totally mixed up. Furroughed fields, of course. Or desert dry terrains usually don't lend themselves to deeper depth. Hardpan (like around the base of trees, or under bleachers, etc...) is where old coins can start at a mere 1", etc...
So it all just depends. You will learn the norms by digging a few targets at various depths, of each site you go to. But if you're in a relicky mindset type hunt spot, then perhaps it won't matter, and you're digging everything conductive anyhow.