The furthest I've ever been able to go non-stop is a little over a metric century. I've done many centuries and even a few double-centuries with short stops.
I like the definition of "within 24 hours" over "in one day" as I've done some 24-hour races where we rode past midnight into the next day. However I tend to think that the ride should be somewhat continuous. Riding 50 miles; taking 6 hours off; then riding another 50 miles seem more like two half-centuries rather than a full century to me.