How often is it okay for my teen to smoke weed. i cant stop them and wonder how often they can do it without permentaly messing up their brain?

Studies show the prefrontal cortex of the developing brain is not finished forming in males until the mid to late twenties. They also show the potential for forming addictive behavior is highest in the teen years on to the 20's whether this is nicotine or weed. I cannot say that any exposure in the developmental period is safe.

As mentioned here at HT, the use of recreational drugs during adolescence can eventuate in emotional and cognitive consequences which can persist lifelong. The extent of the harm is unclear, as artificial drugs laced with Fentanyl are on the scene and potentially lethal. Would urge you to contact drug addiction rehab centers for advice and directions. These are listed online.

Cannabis use has many adverse effects. It is hard to determine how long a use to cause permanent since quantity of use and other mental or addictions may impact people differently. It is important to consult an Addiction specialist. Also of major concern will be risk in MVA due to increased response time