I'd say at least a couple months, depending on how you treat the first marathon. If you race the first for time and they are too close together, there's no time for training. You're either recovering from the first or resting for the second. If you treat the first as just another long training run, then recovery will be quicker.
I tried running two marathons in a month's time. I meant the first as a training run, but I had to catch a flight and Albuquerque is at 5000', so my slow pace was actually quite taxing. Five weeks later, my time was slower than the year before.
Naturally, YMMV. I read about a guy who ran sub 2:20 marathons every month for a whole year.