You can use the same recommendations given for fitness walking for Nordic walking, and get even more benefits. Walking has been shown to improve cardiovascular health, reduce blood pressure, help prevent and treat diabetes, manage weight, decrease depression, and help prevent some cancers. Nordic walking does all this, and improves upper body strength and endurance as well. (This list does not cover all the benefits of exercise.) The most common recommendation for walking for health improvement is 150 minutes a week.
If your main goal is weight loss, you should try to double that. This averages an hour, 5 days a week, but can be done in 15-30 minute increments. The 300 minutes at 20 minute miles is only 15 miles a week. Do more if you want, less if you're getting started exercising or out of shape. Do as much as you can comfortably. If you follow these recommendations, or those of your doctor or other professional or organization, do Nordic walking for the same amount of exercise recommended for normal walking, and enjoy even more benefits.