Within 20 years: i.e. by 2036.
"Eventually, I expect that collapse is likely. The timing is not certain, but because of Liebig’s Law of the Minimum and the very connected nature of our systems today (oil, electricity, food, financial, international trade, Internet, medicine, etc.), it seems to me that this collapse could take place in as little as 20 years. We cannot of course know with certainty, but it seems to me that we should be at least looking at this possibility, and planning accordingly."
Our finite world: Getting started