mbee,
I finished my stamping on April 14th. I think INS-Columbus does the stamping on every Friday and Monday. But for sure they don't do it on Wednesday.
When you go there, take your I-94 receipt,Passport, I-485 approval notice (courtesy copy is fine too), any EADs and APs, and 2 side pictures. They do the stamping usually between 1:00-3:00pm. I will recommend going around 1:30pm.
When you reach there, the guard will take all the papers, and will ask you to sit down for few minutes. Within few minutes somebody wil take your signature, fingureprint and stamp your passport. The process takes 10-15 min. The nice thing in Columbus is there's hardly any applicants in the office, so you get real fast service.
Remember: You will not get any of the papers back other than your passport. So if you need to make a copies of everything before you go there.
Good Luck!!