They don't really 'expire' in the same sense as milk going bad.
But the flavoring itself can change and no longer taste the same, it might fade or might taste completely different.
The nicotine could separate from it and rise to the top.
The nicotine could oxidize resulting in a peppery taste.
Depending on the flavors used, the juice could darken.
VG by itself can get lumpy if left to sit for a long period.
I guess if let to sit for long enough, any microscopic solid matter could eventually collect and become large enough to become visible.
Generally speaking, as long as there is no solid stuff floating in it, it shouldn't hurt you.