I've stored it for a year at a time, but it did mold a bit. I had to scrape the top layer off and then remix the batch. The water from the paint will separate over time, so when you open it up a year later it will have a bunch of water sitting on top. I find it easier to just mix smaller batches.
I just checked on a batch made over a year ago and it's still good. I mix and keep mine in those drywall mud buckets that the lid is always impossible to take off.