How long after back surgery will someone start to feel better?

DEPEND. There are many different types of back surgery for many different problems. Younger individual tend to recover quicker than the older patient. You should speak to your surgeon regarding his/her patient experience for the type of surgery you will or has undergone.
Varies. It varies based in large part on the surgery that is being done. Some surgeries can have an immediate improvement in symptoms, where other surgeries are so extensive it may take several weeks to months. If you feel like you are coming along too slowly, I would not hesitate to discuss this with your surgeon.