Sept. 18 (Bloomberg) -- David Kotok, chairman and chief investment officer at Cumberland Advisors Inc., and Robert Eisenbeis, chief monetary economist at Cumberland Advisors Inc., say they are unsure how long it will take the Federal Reserve's third round of quantitative easing to drive economic growth. Kotok and Eisenbeis speak with Bloomberg's Carol Massar and Michael McKee on "Bloomberg On the Economy." (Source: Bloomberg)