Hi
I have juice that has been sitting in CE4s for almost a year. Two of the clearos died because the mildly caustic juice broke them down over time and one had thick juice it in that heavily gunked up the wicks, so it's going to live in the trash too. Welcome to the wild frontier! No one knows what can happened next!
The only problems I can think of is leaking, juice breaking down the tank, juice tasting weird, juice looking weird.
I don't like to clean, so I tend to designate one flavor for one tank and just keep filing it and hot swapping until the juice tastes like toast or I decide I don't like it anymore. It sounds like you might not be using a carto tank, but if you are and don't know what a "Hot Swap" is, click on the link in my signature.
ETA: For other type of tanks, I still only use one flavor in each, but then throw the clearo or head away when it wears out.