As a general rule most coin size finds are in the top 6-8'' here in the UK,so for the most part they are your everyday digging depth,but of course because of our rich history i would not stop digging at that depth as if it gives a good 2 solid signal then i will carry on digging to find out what the target is either good or bad,would not sleep if had not dug a target and it could have been the holy grail of finds.
When deep hunting on either pasture or ploughed land and in a area that has all the hallmarks of a possible scattered hoard then i use the big fella the TDI with the big 20'' coil on,then however long it takes i will dig down to find what ever the target is once again either good or bad,most hoard as a general rule found over here are in the 15-24'' depth range,which is roughly the length of a mans arm when burying a hoard when he is laying flat on the ground,these figures are of course just rough guidelines but pretty reliable,so its very rare that i would dig deeper.
As they say 'he who dares wins the game' of course this is only my personal opinion on how i detect,but digging to say just 6-8'' when over the years most finds would have been cleared out over the decades by folks detecting from the 60s-80s,so my way of thinking is that one needs to go down just a little deeper to find the next 'possible' layer of finds,which of course would be older still that the top layer.I liken it to peeling the layers of a onion,by using say a more modern detector and slightly large coil you are able to go slightly deeper after all the junk and shallow targets have been found,so a larger coil can be used to your advantage.