Cessna, what you write in your rebbi’s name is ridiculous. It is not a chumra, it is shtus. As I wrote earlier and you seem to have ignored, a 10-hour flight is no more subject to delays than a 1-hour flight. Whatever buffer you determine is sufficient for a 1-hour flight should be sufficient for a 10- or even 18-hour flight as well. On the contrary, as Gadol pointed out, longer flights actually need less of a safety buffer than shorter ones.
Driving is different, of course; there the risk of delays is on the road, not while sitting in your own home or driveway, so the longer the road the more likely they are.