the rate of speed you're doing is 250 miles every hour. So in 1 hour you will travel 250 miles, and in 2 hours you will travel 2(250) = 500 miles. So it will take you 2 hours to travel 500 miles if you're going 250 mph. :)
The famous equation to remember here is d = st where d = distance, and s = speed and t = time. So if you put 500 for d and 250 for s, you are just solving for t which in this case is hours. 500 = 250t.
Hope that helps!