What's an ex worth...

Those of you who strive for precision in language will appreciate a conundrum that has monopolised my thinking since I heard news of a scheme to rehabilitate 'ex-offenders.' My alert neurons quickly identified the essential ambiguity in the announcement, and lead me to ask what, exactly, is the difference between an ex-offender and an offender?
The prefix 'ex' clearly connotes a departure from a former state, but in what units is the departure to be measured, and what value must be attained before a definite departure can be said to have taken place? Does an offender become an ex-offender merely through the passage of time, or must some degree of reform be present? Does an inveterate lag who has not, for pure want of opportunity, offended for twenty years qualify as an 'ex-offender'?  Is he as much an ex-offender as a convert from sin who has wholeheartedly devoted herself to good deeds since last hearing the cell doors clang behind her a year ago?
The forgoing assumes ex-offendership to be a quality that can be possessed in degrees; but could it be a binary quality that is possessed and possessed in full only when some threshold of absence from offending is reached?
These questions might seem abstractions appealing only to the pedant or the philologist (I assume the difference), but a moment's thought will reveal their intensely practical importance. How will the organisers of the scheme for ex-offenders decide the eligibility criteria for participating lags? How will politicians decide whether to fund the scheme? How can the success of one scheme be reliably compared with that of another if they adopt inconsistent definitions of ex-offendership? These and similar questions are the all-consuming burden of the true philosopher, and yet still the wife wants to know when that decorating's going to be finished. I ask you.