A 21 day fast should be long enough for your goals. Its been 16 days and I've lost a lot more than 20 lbs, so you should be fine. The article I read was referring to having a clearer mind and improved brain function - that is what requires over 21 days.
I've intentionally fasted once for 5 days, that was 3 weeks before I began this fast. However, I often have bouts of Depression that lead me to "fast" unintentionally and I usually go 1 - 3 days without eating.