How soon will I be able to lift weights after having surgery for the slipped discs in my spine?

1 to 3 months. Your surgeon can guide you in your specific situation, but generally you need to let the soft tissues (disk, muscle, ligaments, skin) heal. That takes several months. Premature heavy lifting might cause another disk injury. Low impact aerobic activity helps recovery.
Never. Talk to your doc but if u are going to resort to surgery u should probably never lift anything over 15 pounds again.
Depends. It varies quite a bit based upon the nature of the surgery done. A fusion would take considerably longer to recover from than a micro-discectomy, for example. You would certainly want to check with your surgeon as it could be anywhere from 8-12 weeks typically.
Depends. After spinal surgery, it is not uncommon for patients to be able to lift weights once cleared by the surgeon. Simple disc excision cases and some fusion procedures may allow a significant functional return with sometimes minimal lifting restrictions (up to 40-50 lbs). More severe pathologies may require permament restriction of lifting.