How many hours are in 721 miles?
There are no "hours" in any distance.
If the question is really "How long would it take to travel 721 miles?" then the answer depends on the average speed, which depends on the mode of travel.
A one-way flight from London (LHR) to New York (JFK) takes 10 hours and 25 minutes. Flight one: LHR to MAN on Virgin Atlantic 3041 takes 1 hour. Miles: 151 Layover at MAN for 2 hours and 15 minutes. Flight two: MAN to JFK on Pakistan 721 takes 7 hours and 10 minutes. Miles: 3421 Price: $503 Miles: 3572