Well, I think that some folks might be surprised that I chose two hours in the poll. I give my wife a time that I expect to be home (or on the phone) and then a separate time to call the authorities to report me overdue. That second time is rarely more than two hours later. I make a real hard effort to be home, or at least back to cell phone range, by that first time. I also leave her a map and a description of what I'm doing.
In many years of responding to SAR callouts, I never minded the all-so-frequent "turnaround" call on the radio or cell phone after starting out from home in the middle of the night. And I don't recall hearing a complaint on that point from any other responder or law enforcement officer -- it's part of being a responder. Delaying the overdue report makes the search harder and puts someone's life at risk.
Maybe some of my conservatism on this point reflects the fact that I go solo all the time. Since I'm the one making the route plan, setting the pace, and making the decision when to turn back, I have a pretty good idea what sort of thing would delay me seriously. And going solo means that that "sort of thing" needs an early response, not a later one.