Airglow (and it's variation with time) is actually quite a complex subject, and "airglow" actually refers to a whole host of atmospheric emissions. Most commonly though, the green airglow you see in photographs is the atomic oxygen 557.7 nm emission which, incidentally, is the same one that appears in the aurora (but caused by a different mechanism).
It seems to create quite a stir these days (and with many misconceptions) mainly because of the high-sensitivity of DSLRs being able to consistently record the fine structure and colouration. But it has always been there.
Actually, it is not merely "common", it is there 100% of the time. Airglow is the main reason the sky (in between the stars) is not completely dark. There are other factors (e.g., unresolved stars/galaxies, zodiacal light), but airglow is the dominant component. Put your hand up to the sky on a dark night and away from light pollution - you will see a silhouette. The background "glow" is airglow.
But yes, it's intensity does vary on both short (second to minutes) and long (decadal; solar cycle) time scales, with the background sky brightness being significantly brighter (0.4 mag/arcsec^2 across the optical V-band) during solar maximum.