How effective is sertraline for chronic OCD? I’m a long term OCD sufferer and was prescribed this...

In my opinion, not very effective.

It may be surprising, but significant and current research indicates that the antibiotic minocycline has shown to be significantly decrease symptoms of severe and chronic obsessive compulsive disorder in multiple double blind studies and has been prescribed to treatment refractory patients with OCD.

Hi, Phil!
Sertraline (Zoloft) is one of the few antidepressant/anxiolytics that's FDA approved for the treatment of OCD. It has a review rating of 7.6/10. That being said, everyone responds differently to this medication so it may or may not be effective for all.
These reviews may help address your concerns:
https://www.drugs.com/comments/sertraline/for-obsessive-compulsive-disorder.html
Best regards and I hope you're feeling better soon.
Wildcat
PS, Yes, 200mg/day is the standard recommended maximum dosage.