How do you know when you've smoked 1 regular cig's worth of a "Blu" e-cig?

I wanna try electric cigarettes but i cant seem to find out how you know when you've smoked 1 regular cigarettes worth of it since each cartrige has like 7 cigs worth in it. And idk about other smokers but if my regular cig's didnt burn down to the filter i wouldn't know to stop lol. Anyone smoke "Blu" e-cigs or any other kind of electric cigs and know the answer? Thanx a ton ^-^