Your question is incomplete as you don't say what your target rate of descent is. Are you thinking of descending at 10,000 fpm, or 100 fpm? In the former case you need one minute, in the latter 1 hour and 40 minutes!

As the other poster said, know your speed in nautical miles per minute. Normally you see the speed in nautical miles per hour, i.e. knots. There are 60 minutes in an hour, so divide by 60 to get nautical miles per minute. For example 60 knots is one mile per minute; 90 knots is 1.5 miles per minute; 120 knots is 2 miles per minute, etc. In your example 300 knots is 5 miles per minute (300/60).

Knowing your speed in nm per minute is very useful because all climb and descend rates are expressed as feet per minute. So you have a common basis – you consistently work in minutes not hours.

Then look at the altitude you need to lose and the rate of descent you are planning to use. In your example you need to descend 10,000 feet. Assuming a 2000 fpm descend rate, as suggested by the other poster, you need 5 minutes. Don't try to remember formulas – just think of what Feet Per Minute means, and then it is obvious that you need to divide the altitude to lose by the rate in order to get how many minutes it will take.

So you are flying along at 5 miles per minute and you need 5 minutes for the descent. Bingo – that's 25 miles out.

Another variation of the same calculation that you need to perform in real time is this: I am 25 miles out. My speed it 300 knots, and I need to descent from 27,000 to 17,000 NOW. What rate of descent is appropriate?

Again you immediately know you are going at 5 miles per minute. You are 25 miles out, so you'll be there in 5 minutes. So you need to lose 10,000 feet in 5 minutes. Again simply by knowing what Feet Per Minute means you know to divide 10,000 by 5 and bingo! You need a 2000 fpm descent rate.

I don't like remembering formulas because if you forget or mess up the formula even a little bit you get the wrong result. Best is to understand the calculation and learn to do it fast. Also, I make a point of noting what the halfway point is in order to see if I lost at least half of the altitude when I get there. Nobody else does that, but then again I am weird – I have nothing better to do while jogging on the treadmill but calculate the calories I burn based on my speed and time I've ran so far!