I'm careful not to tear up the ground, regardless of the size or depth of the hole. I use the same technique -- hinge plug, regardless of the size, but keep it as small as possible based on the depth of the target.
I give up on holes all the time, and fairly quickly. Its about efficiency and cutting losses for me. I may have missed a deep iron strongbox full of gold coins, but I'll take my chances on that one. Basically, I cut my losses when the target isn't as expected, based on what the detector was telling me before I dug. Targets beyond expectation are almost always large ferrous, but I've probably missed a few silvers as well. I'm betting an attitude of efficiency pays off in the long run.
Now, keep in mind that if the initial signal is difficult, I'll give it more time, or I think it has to be a spill, affected, or so forth. These difficult ones can be a deep silvers on their sides. It all depends on the initial signal vs expectations for me, and its hard to describe. With the E-Trac, there is almost always a target when there is a repeatable signal, that doesn't mean I go for them all. And, I don't think I've dug a hole deeper than a foot. I rarely dig holes deeper than 6 inches.
A couple of things to keep in mind. On very deep targets, the signal can be lost once some of the dirt is removed (dirt is more conductive than air). When this happens, I still dig another inch or two. Don't give up on these. Also, I'll refill the plug to get these to come back sometimes.
Finally, when digging deep holes, all that dirt expands. It won't fit back in the hole. Pack it in with the heel of your boot or discard it in the woods. Don't do like an idiot that was hunting one of my sites was doing, and just leave the plug sit above the surrounding ground by an inch cause the dirt wouldn't fit, leaving the park people to get in my face. You really need to make the effort to make your work invisible. Practice in your own yard first might be a good suggestion.
HTH