How long does it take to recover (say 90%) from back surgery?

Depend. The recovery from spinal surgery depend on the nature of the operation. For discectomy, traditional open vs micro discectomy/microendoscopic (miss) , miss is shorter. Most patient return to full activity within 6 weeks. With a fusion procedure, recovery depend on number of levels involved and how vigorous the rehab program you actively participat in. It may vary anywhere from 12 wks to many month.
Varies . It varies a lot based upon the type of surgery you had. Some surgeries have very little recovery at all, even as low as a few days. Others, such as fusions, may take even up to 6 months to fully recover. If it gets beyond that, you may need some more testing to make sure nothing else is going on.