In an ideal world, the answer to that question would of course be none: teenagers should use no illicit drugs of any sort. But here on planet earth, 36% of 2012 High School Seniors had used marijuana over the past year, and 6.5% used the drug on a daily basis*. So, how should you distinguish between harmless marijuana use, worrisome marijuana use, and harmful marijuana use? First, any use of marijuana puts the teenager in some amount of legal jeopardy, and entails at least the risk of thinking problems like memory loss, poor motivation, and lethargy. And some teenagers have their lives destroyed by marijuana, although that is rare. To my mind, then, there is no use of marijuana by a teenager that is harmless. But many, if not most, teenagers marijuana users take the drug with little evidence of problems in school, relationships, or physical or mental health. I would characterize this use as "worrisome," in that it might lead to serious problems, even if it hasn't yet. Harmful, marijuana use, by contrast, has the obvious pattern of social isolation, paranoia, school absences, etc. Teenagers who have been addicted to marijuana or anything else, or have a mental illness, put themselves at grave risk if they use the drug. I never give permission for teenagers to use marijuana, and neither should parents. But I don't call 911 either, because we all must distinguish between behaviors which are immediately life-threatening and those which are potentially harmful, and manage them accordingly. The good news here is that if you are addressing "How much marijuana is too much" with your teenager, you have a dialog going. Use it! Without lecturing, ask your teenager how marijuana affects her. Talk about your concerns. Ask her what, if anything, concerns her about marijuana. In the absence of any signs of harmful marijuana use, you are better off registering your concerns and keeping the subject open, rather than declaring an emergency where none exists.
*Johnston, L. D., O'Malley, P. M., Bachman, J. G., & Schulenberg, J. E. (December 19, 2012). "The rise in teen marijuana use stalls, synthetic marijuana use levels, and use of 'bath salts' is very low." University of Michigan News Service: Ann Arbor, MI. Retrieved 01/21/2013 from http://www.monitoringthefuture.org