There are many occurences in the archives of the term 'hack', used in both complimentary and uncomplimentary senses, with the first known use in 1979. That it made its way into a written document suggests that it was reasonably commonly used at the time, and other occurences within the archives suggest that the term, in both its complimentary and uncomplimentary senses, has been fairly consistently used.
However, since our definition is none of the commonly-used ones, this makes it difficult to determine how long hacks have been around for. There are actions in the archives that many hacks would have agreed with, and signs of the cynical humour most hacks recognise and appreciate, but this, while indicative, is not proof. It is one case where the archives are insufficient.
It is known, therefore, only that hacks have been around since 1996, but were likely around for many years before then. Since then, each year has added another few to our numbers. We have never been numerous, but there have usually been enough.