Terminal is the term used for hair at maximum lenght. Ones terminal lenght vary person to person, most determined by the persons genes.
The growth cycle of a hair differs vastly between people, ranging somewhere between 2 to 7 years before the hair falls off. Terminal hair lenght would therefore in theory be the lenght your hair grows in that period of time. But some people never get to terminal lenght due to damage and breaking of the hair at the ends, making the hair shorter.