It varies. We've been through two and another is scheduled in a few weeks. They'll tour your home, ask you questions about your marriage, your childhood, your beliefs, your parenting and discipline. They may ask about safety issues such as if your medicines are locked up, guns, fire extinguishers, smoke detectors, etc.
I was really nervous before both of ours but they weren't bad at all. Both lasted around 30 minutes or so. The one coming up, the sw said she'll be here for 2 hours.

This one will be a state adoption study. We've done a private study and a state foster study before.
Paperwork they usually need involves paycheck stubs, taxes, birth certificates, marriage licenses, divorce certificates, etc.
Oh and for our foster study, if we had a septic system, they'd need to ahve checked it, but we're on city water.
Good luck!