First of all, I485 backlog went down in January, February, and March. There isn't any data for April yet, but I expect to see something similar.
If I remember correctly, there were 2-4 approvals on rupnet daily since the beginning of the year. That resulted in 45,000-55,000 processed applications per month. That's across all SC.
I know it's wrong but let's assume that all SC's have the same acceleration. This week NSC seems to be on fire. There were 12 approvals on Mon and 5 approvals on Tue (so far). 12 is most likely an exception but if we see 4-6 approvals on rupnet daily that would mean 75% increase in processed applications. Therefore, all SC's would be processing 80,000 to 100,000 per month. Those are numbers that they already achieved back in early 2002 and I doubt they can go much above them.
With an average of 50,000 new applications per month, the backlog reduction rate would be somewhere around 40,000. Therefore, with current backlog of 1.2mln it will take them 30 months to clear it completely. Now, they don't actually need to clear all 1.2mln. To bring the processing time down to 6 month, they need to take care of 1,200,000 - (50,000*6) = 900,000 applications which is just under 2 years.
I admit it's just a mix of scientific guessing and wishful thinking that is based on very rough assumptions but I'm *happy* that NSC is finally moving at some reasonable pace.