How long does the numbness and pain in left leg last after lower spinal surgery?

Totally uncertain.: Could be damage to the nerve root itself from the surgery. It could burn itself out over time , but not a guarantee.

Depends: Determining the cause is important. Notify your surgeon to rule out hard wear pinching the nerve or evolving seroma, hematoma, etc.

That can vary as to : The original diagnosis & whether it was present preop & for how long as well as if there are other underlying medical issues like diabetes, neuropathy, thyroid disease etc. -sometimes nerve tissue does not fully recover even with correct surgery & then there are less successful outcomes due to complications or to less than optimal surgical technique and procedure. So, the answer is very variable.