Been on Klonopin 0.5mg for 6 months for PTSD. Also on 200mg Zoloft (sertraline) and in therapy. When should I start to taper off the Klonopin and how fast?

Depends. Staying on or stopping meds for PTSD should be tailored to your unique situation and discussed with your prescribing doctor. Your therapist should be aware of any med changes as well. That said discuss with your doctor to make sure this is the right time for you to stop Klonopin (clonazepam) and let him/her guide you on how best to stop. Hope this helps.
Clonazepam taper. You might need to consult with your doctor, but is you are back to an acceptable baseline and symptom-free, it is generally advised to continue the Sertraline and gradually taper your Clonazepam. It is very helpful that you are in therapy, as there are very effective interventions for PTSD on that treatment modality.