The flavor dies out. That's the bigest sign. You can puff the same filter for a while and the flavor will still be there (though less prevalent). I'm betting the nicotine is gone by the 10-15 puff mark. Depending on how deep your pulls are. The first drag off a filter is very nice. They sorta fizzle out from the first drag, as they should. If you're a smoker, a good thing would be to forget it and just place a new filter when you feel the 'urge.' Assuming that the device is no longer satifying you.
It is quit nice though, was puffing throughout the day at the office.