Bacteria will not grow in juice. The only breeding ground - drip tip. I almost never wash my tanks. I wash drip tip at least twice a day. I can grab a tank which was in a drawer for 2 months (filled with juice) and vape it - no difference with freshly filled one. But I am DIYing. I know my juices are clean (I buy only colorless flavorings). I remember two juices from the time I was buying them: one had separation of liquids, another one developed sediment after a few months of storage...
EDIT: of course, if you leave filled tank for too long (like six months or more) nicotine oxidation could be noticeable (just theoretically speaking, never tried it with my tanks).