What length mortgage is typical? There is no easy answer to the question of what length mortgage is best, but traditionally, an average mortgage length lasts a period of 25 years. However, this mortgage term can be longer or shorter.Mortgage lengths have been stretched over the recent years.here is no easy answer to the question of what length mortgage is best, but traditionally, an average mortgage length lasts a period of 25 years. However, this mortgage term can be longer or shorter. Mortgage lengths have been stretched over the recent years.