I would keep a circulation pump going to keep it aerated. Keep a tight fitting lid on your water to prevent evaporation, since the biggest thing I would worry about is having your salinity creep up before you use it. (That's probably what is happening to your RO water TDS too.)
If you check the water to make sure the salinity is correct, I think you could let it sit for months with no issue.
I use a salt mix that mixes up almost instantly, so I make my water about 10 minutes before I use it.
EDIT: Now I see that you said your buckets were sealed. The only thing I can think of that would increase your RO water TDS as it sits would be leaching from the bucket into the water, evaporation, or dust settling into the water.