The current trend for Boston Marathon qualifying cutoff for the 2016 Boston Marathon is BQ minus 1 minute 47 seconds.
The qualification window for the 2016 Boston Marathon began on September 13, 2014. Of the 25 “Top Qualifying Races”, ten have already occurred this year. Data has been analyzed for these races so far (data for the Berlin Marathon was not available for the analysis).
Source (including data and methodology):