48" x 13" x 1" = 624 cubic inches
1 gallon (U.S.) = 231 cubic inches
624 cubic inches / 231 cubic inches = ~2.7ish

so, I'm estimating around 11 quarts minimum.
probably a bit more, depending on how it compresses, and how much/whether you sift it.
I didn't screen out too much via sifting (I can't remember what size bag I had off hand, I think I lost less then ~20% of volume to shifting), but I also broke up some of the chunks and forced them through, so if I was just using the shake-shift and dump, I would have ended up with much less.
1 gallon (U.S.) = 231 cubic inches
624 cubic inches / 231 cubic inches = ~2.7ish
so, I'm estimating around 11 quarts minimum.
probably a bit more, depending on how it compresses, and how much/whether you sift it.
I didn't screen out too much via sifting (I can't remember what size bag I had off hand, I think I lost less then ~20% of volume to shifting), but I also broke up some of the chunks and forced them through, so if I was just using the shake-shift and dump, I would have ended up with much less.