Two reasons for the light to flash.
1. it will flash after you have been inhaling on it for about 5 seconds. This is a cut off, on most the light will flash for about ten times, this is to let you know the power has stopped until you inhale on it again next time.
2. the battery needs to be recharged. The light will usually flash as soon as you start to inhale and will flash for a much longer period of time, as long as 9 seconds or 30 or so flashes. At the same time you will notice there isn't as much mist being made anymore. Time to change the battery.
You will get used to what your penstyle is telling you quite quickly and know what needs to be done, a battery needs changing, or a cartridge needs changing, you will know very soon how to tell the difference.
I always carry at least one spare battery with me and try to make sure the battery I'm using is fully charged before I leave the house. I've always got at least one spare cartridge with me as well. Thinking about it, a spare battery and a spare cartridge are about the same size as the cigarette lighter I don't have to carry anymore.