The duration of a stock is the average of the times until its cash flows are received, weighted by their present values. The most popular model of duration uses dividends as the cash flows. In vernacular, the duration of a stock is how long we need to receive dividends to be repaid the purchase price of the stock. If a stock doesn't pay dividends, other methods using distributable cash flows, may be utilized. The duration of an equity is a noisy analogue of the Macaulay duration of a bond, due to the variability and unpredictability of dividend payments. The duration of a stock or the stock market is implied rather than deterministic. Duration of the U.S. stock market as a whole, and most individual stocks within it, is many years to a few decades. A nominal value, assumed in many analyses, would be 20-30 years, analogous to long term bonds. Higher price/earnings and other multiples imply longer duration. Duration is a measure of the price sensitivity of a stock to changes in the long term interest rate, i.e., the longer the duration, the more sensitive the stock is to interest rates. In U.S. stock markets, an SEC rule adoption in 1982 (rule 10b-18) that allowed discretionary stock buybacks has distorted the calculation of duration based on dividends since at least the early 1990's. The rule change had no ascertainable impact on duration, but duration now needs to account for all cash distributions including buybacks. Duration in the discounted cash flow model The present value or value, i.e., the hypothetical fair price of a stock according to the Dividend Discount Model, is the sum of the present values of all its dividends in perpetuity. The simplest version of the model assumes constant growth, constant discount rate and constant dividend yield in perpetuity. Then the present value of the stock is where P is the price of the stock D is the initial dividend amount r is the periodic discount rate (either annual or quarterly) g is the dividend growth rate (either annual or quarterly corresponding to r) The requisite assumptions are hardly ever true in perpetuity, so the computed value is highly hypothetical. In the Discounted Cash Flow Model (DCFM) of security analysis, the value of a security is the present value of all its future cash flows including interest or dividends and the implied cash flow of the residual value