I had my FP on a weekday too. Landed up about 15 minutes early. At the door, the security validates that you have your appointment letter and ID. You then join a line of people and fill up a form attached to a clipboard - basic information like name, age address of applicant etc. I had about 15 people ahead of me and line moved at a decent pace. I landed up on the front of the line in about 10-15 mins at which time a lady at the desk validated my letter (stamped it), my photo ID and GC. I was then given a number and asked to wait in this big room with several chairs. At the end of the room were 4-5 desks where USCIS folks were finger printing applicants. They call your number - sometimes 2-3 numbers together. You walk up, show your GC and letter, they finger print you and return your letter and GC and you are done.
The wait for me was about 30 mins and the actual FP was about 5-10 mins.
The entire process once you get to the building was about 50 to 60 minutes.