Often this measured as the simple average of monthly growth rates, but investors often prefer to measure it as CMGR (Compounded Monthly Growth Rate) since CMGR measures the periodic growth, especially for a marketplace. CMGR is a more accurate metric for evaluating changing financials.
Using CMGR [CMGR = (Latest Month/ First Month)^(1/# of Months) -1] also helps you benchmark growth rates with other companies. This would otherwise be difficult to compare due to volatility and other factors. The CMGR will be smaller than the simple average in a growing business, but more helpful for analysis.