Yeah, it gets gummy and unvapeable. And, usually, the juice will oxidize: it gets darker and the nicotine gets peppery.
High VG juice, even if it's kept in the dark, will absorb a lot of water, with obvious negative effects.
I've had juice in a tank go bad as quickly as a week. I assume that's because I use very little flavor in some of my juices, so they're at 95% VG.
It depends on the tank, too. Some allow more air in than others. I have a Calyx that keeps juice in perfect condition for months. It's weird.