I have been collecting rainwater this summer too..
I collect it in 5 gallon buckets, fill gallon jugs from the buckets, then if I have any left over, I seal the bucket with black plastic (garbage bags) then put the bucket of water in a dark closet..
without light, nothing should grow in the water..
no algae or bacteria or larger lifeforms..without sunlight to fuel the process, there should be almost nothing living in your rainwater..
so the answer is, if stored in darkness, rainwater should be "good" forever..
the water stored in my gallon jugs sits out in the open, but not in direct sun..even so I wouldnt worry about that water, because its quite pure and there isnt much "food" in the water to support much of anything..and even if some algae grew, so what? that wouldnt hurt anything..
organics in the water wont hurt CPs, its the mineral hardness that is the problem..rainwater wont ever get mineral hardness by itself..
so IMO, raiwater is good pretty much forever! or at least several months, or a whole season, which is good enough!
Scot