I needed the harness extension, my guess is that most recent 12is builders will need it. Mine is a dual Garmin setup so I'm not familiar with what may change with other panel configurations but it really wasn't even close and I think most will need it.
As to the time frame, it's pretty easy. My 13 year old daughter did 95% of this SB with close guidance over the course of 4 evenings. An experienced builder with a longer attention span would probably do it in a couple hours. This is with the forward top skin removed.
Nate - Salida, Ca
RV-12is - Prepping for Inspection