I have one of those blue 50 gallon drums with water in it. In my last place I kept it in my garage up on blocks, to keep it away from the concrete floor. I had it stored there for 8-9 years without changing it. When we filled it up initially with a garden hose, we included 1 cup of bleach. That was the amount we had heard at the time and it seems like a bit much. But when it came time to move, I had to siphon it out. It didn't taste the best but I didn't get sick either. I suspect if it was oxigenated by pooring it back and fourth, it would help quite a bit. I also have drink mix for this purpose.

I don't know if I would leave it forever, but I personally don't think 5-10 years is a big deal if your source is reasonably clean, your storage container is sanitized, and you add some bleach.

My .02.