How long will a 20,000 mAh power bank typically last?

If you need to charge your phone repeatedly while away from an outlet, a big power bank could be a good investment.

If you need to charge your phone repeatedly while away from an outlet, a big power bank could be a good investment.<p><img src="https://www.engadget.com/img/gallery/how-long-will-a-20000-mah-power-bank-typically-last/intro-1790271761.jpg" /></p>