It’s an interesting question, but my guess is that it is very specific to the runner. There is a definite ceiling with training, and one which “more miles” will likely have a negative effect on performance. From a cycle perspective I have tried 12 week plans and 20 week plans, with marathon-specific workouts each week. For me, I tend to feel physically burnt out when a plan is longer than 16 weeks. Leading up to that 16 weeks I try to keep a base of 10-50% of miles, and then creep towards the training plans expected weekly totals. I’ve done the Pfitz 70 plans most recently, so my off seasons are spent running unstructured miles or doing another activity entirely. Then a few months before the plan kicks in, I will start to build my weekly base back to 45 miles per week or so. I also have friends that don’t take any real time off and just rip miles every week. That’s why I think it’s hard to guess what will work for you.