As I said, she had been running for just about 4 years. She was typically racing local 5ks with a half marathon once a year and had gotten to the low to mid 30s pretty consistently, so once she signed up for Chicago, we set 50'as the peak mileage goal because we had almost a year to get there, so that included an entire training cycle before he marathon cycle. And with marathon cycles, getting to 50 really isn't hard since your mideeek workout can get to 10 miles even if you're being conservative in your training. That and a long run of 16-20 gets you half way there.
So we took her up from about a typical 32-35 to an occasional high bear 40 to a consistent for an intermediate cycle and then brought her up to a peak of 50 miles for Chicago.
Note that I said peak. I think we cycled through weeks of 40-42, 45 and 50, so it wasn't a constant stream of 50 mile weeks for her. Just a few (3 or 4). Again, we reached that number based on her relative inexperience, the amount of time we had before her ultimate goal (a lot), her previous peak (not a lot, but consistent and had been near 40 in the past), the length of the goal race (a lot), her relative good health and economic running form (definitely a shuffler as opposed to my loping stride, which makes her less susceptible to some injuries), and her psychological need to feel like she was well prepared.
I don't necessarily believe that there is a bare minimum required mileage, but if the circumstances allow it, I think a peak of 50 miles is a good goal for most folks who want to get the lion's share of their possible success in their first marathon. It may not be viable, and different people benefit from increased volume differently, but when it comes to the marathon, mileage is king.