Tubby -
I can only tell you what my experience is. Your mileage may vary (YMMV).
I've had one 50 lb. bag since June 15th. I have 3 mature chickens and 11 8-week-old chicks. I've got about 1/4 left now. I use the DE in my deep littler, and in their feed. So, about one 50-lb bag every 3 months, or about 200 lbs. per year. Maybe less in the winter.
Here's how I store the DE: I put the bag it comes in into another large, heavy-duty plastic construction bag, then put that into a large Rubbermaid-type garbage bag. I think this keeps it contained, and safe from moisture.
Hope this helps!