I can't remember exactly how long it took as I started this process in 2010, but I remember it being close to a year long process. Granted, the NFA wait times were different then and paper forms were our only option.
As for the process, it makes sense that it will take forever to get done. If you think about it, you have to wait for the KRISS to change the barrel, and then submit a form 2. Then you have to wait for an approved form 3 to transfer it to your dealer, then an approved form 4 to take possession of it.
ETA: Actually looking back at my emails with KRISS USA, it took a little over 4 months from the time I sent my rifle in to the time it was shipped back to my dealer. For me to actually take possession of the SBR it took a year and 3 months, but this was back in the day of paper forms and Major Malfunction being my dealer.