I've left chemical in mine for at least a week (prepped it, went camping, didn't use it, went again next weekend). My advice is to use Aqua Chem. It's way better than the West Marine store brand, at the very least.
I leave the flush tank with whatever's in there until I think it might freeze, and top off before leaving / after arriving / once it runs out.
1995 Toyota 4Runner SR5 4WD 5 spd
2002 Eagle Summit 12