I just resolved this exact problem on my 99 Legacy Outback. The reason is that when you are adding fuel to the tank, the vent is clogged. So when the fuel is going down the neck, the pressure backs up, and the air has nowhere to go.
You should have a charcoal box on the rear pass side under the bumper. There are 3 hoses that go toward the front of the car. 2 large, 1 small. Take an air compressor and a blow tip and blow air through the 3 lines. There is a "clog" in one of those lines. Make the "connection" to the hoses tight enough that the compressed air will go throught the line. Then one that goes to the tank will build pressure and release some vapors.
It is possible you may have a damaged hose as well. I checked all of my hoses for breaks and couldnt find any.
I went for 2 months before I had the time to troubleshoot this problem. It would take 8-10 minutes to fill the tank. Now, it works just as normal.
Hope this helps.