Just my opinion of course, but (like drugs and alcohol) putting a reasonable and generic metric on it is really hard, so I would instead ask generic questions that I would ask about other things like:
1. Do you find yourself craving it?
2. Do you continually feel the need to increase your intake? (i.e. developing a tolerance)
3. Would you be embarrassed if a like-minded friend knew about your habit?
4. If you were suddenly cut off from it for a few days, how would it make you feel?
3 is really a societal measure - many addictions, like tobacco addiction and to some measure alcohol addiction, or caffeine addiction, are well accepted by the society and usually carry little shame to the addict.
1. Do you find yourself craving it?
2. Do you continually feel the need to increase your intake? (i.e. developing a tolerance)
3. Would you be embarrassed if a like-minded friend knew about your habit?
4. If you were suddenly cut off from it for a few days, how would it make you feel?