There are no fixed rules for determining the duration of the forecast period. However, choosing a forecast period of 10 years, for example, will not be meaningful when individual cash flows can only reasonably be modeled for four years; see
Cashflow forecast.
The number of forecasting years is therefore to be limited by the "meaningfulness" of the individual yearly cash flows ahead.
Addressing this, there are three typical methods of determining the forecast period.
Based on market characteristics: Determine a forecast period by choosing a number of years based on the characteristics of the market. Companies in established and well known markets are better suited towards longer forecasting periods than those opening up a new market, or
startups.