I did a long over-water trip with a friend in his PA28 last Summer, to the Faeroes (great destination!)
He has a clockwork hour-timer rigged up in front of the passenger seat and briefs the pax that it's their duty to keep an eye on it and holler when it does an hour. Then he changes tanks. Works well when you're using the limits of endurance of the a/c. Incidentally, at the power setting he normally uses, we wouldn't have had an alternate if we'd been unable to land at Vagar. At my suggestion, we used about 150 RPM less and would have made it back to Scatsca with reserves...
TheOddOne