How many years is generally accepted as a reasonable timeframe for ratification?

The timeframe for ratification of amendments to the U.S. Constitution is not explicitly defined, but a common practice has been to allow a period of seven years for ratification. This timeframe was established in the Equal Rights Amendment proposed in 1923 and has been referenced in subsequent amendments. However, some amendments have been ratified long after their initial proposal, indicating flexibility in the ratification process.