Had lap surg to remove fluid from ovarian cyst thought to be irritating sacral nerves causing sciatica pain. How long post op should it take for nerve pain to resolve? Spine already cleared by mri

I doubt that. A large ovarian cyst of more than 6cm may cause significant pain, but the lumbo-sacral pain would be referred, rather than direct neurapraxia. If your pain hasn't resolved following surgery, you need to have further evaluation and treatment on your back.
Hard to say. Surgery will have some post-op issues for about 2-4 weeks. After that is should be getting better. If MRI cleared your spine maybe you want to get EMG/NCS to see if its something else as well like a neuropathy.