That depends a great deal on a number of factors, including pad quality and vehicle weight. But I would say a swag might be 20k. Some manage to go through a set in less than 10k, others, on rare occasions, get close to 40k.
Using engine braking to slow you down makes a great deal of difference in longevity. This can also reduce brake fade in more severe conditions.
Something that a lot of people don't realize is that their brake pad wear rate is greatest when the pads are new. As the pads wear, the rate of wear slows, so that when you are at 1/2 the pad thickness, you have more than 1/2 of the pad life remaining.