I sent my grandpa a growler full of my Blonde Ale a while back, I filled it to the tip top using nothing but a picnic tap and put one of the better quality caps on it, (one that I'd used a hundred times I might add) and set it in his fridge. He forgot about it, as did I, and 6 months later he opened it (thinking it would be flat) and drank it. Said it was carbonated fine, and another family member had some as well she confirmed it was carbonated "good" too.
Now I'm not saying they'll all last that long, and I'm not saying you won't see a loss of quality either. My thoughts on storing growlers is, "They'll keep a lot longer than they'll last" if ya know what I mean.