Here is a quick summary how the timetable went from paying immigrant fee to actual delivery of GC in my case:
Paid Immigrant Fee: 2/17/2015 -> Status went to Accepted in ELIS
POE 2/18/2015
Status Optimized 2/20/2015 and In process (Waiting for officer to pick up for further processing)
Status Optimized 3/23/2015 and In process (We are producing your new card for your Form OS155A, Immigrant Visa and Alien Registration)
Status Optimized 3/23/2015 and In process (We ordered your new card)
Status Closed 3/30/2015 and In process (We mailed your new card to the address you gave us.)
Status Closed 3/30/2015 and Closed (The Post Office picked up mail containing your new card.)
So we went quite fast to Optimized but the actual card production did not start that instant. Received that card on 04/01/2015 (how appropriate).
Paid Immigrant Fee: 2/17/2015 -> Status went to Accepted in ELIS
POE 2/18/2015
Status Optimized 2/20/2015 and In process (Waiting for officer to pick up for further processing)
Status Optimized 3/23/2015 and In process (We are producing your new card for your Form OS155A, Immigrant Visa and Alien Registration)
Status Optimized 3/23/2015 and In process (We ordered your new card)
Status Closed 3/30/2015 and In process (We mailed your new card to the address you gave us.)
Status Closed 3/30/2015 and Closed (The Post Office picked up mail containing your new card.)
So we went quite fast to Optimized but the actual card production did not start that instant. Received that card on 04/01/2015 (how appropriate).