That really depends on your goals and training background. Generally, I recommend 16-20 weeks or longer.
The bigger the leap in performance you want to make, the longer you'll want to have a dedicated training plan.
Our plans assess your strengths and weaknesses and write custom workouts to address your weaknesses before starting the race-specific training. My recommendation is a plan of 16-20 weeks.
Likewise, if you're training for your first marathon or half marathon, the longer we have to slowly build your mileage and workouts, the safer we can make your training plan.
Plans of 12 weeks or shorter should only be chosen if (1) you're already training for the event and you want something more custom; or (2) you are an experienced runner.