If duration buffing was based upon the entire chant duration previously (I wasn't in the beta, so can only go by what I've read), it sees like they changed it to be based ONLY on linger duration (yes, that would be a 66.67% nerf).
I created a godlike chanter with 20 INT and selected two buff chants to start, then waded into the first fight with a couple wolves. Using a stopwatch, I observed that the first buff fell off after 7 seconds, 1 second before it was reapplied. With a 20 INT, my duration bonus was 48%, so with a base duration of 4 and linger of 2, I can only assume that the linger was being extended by 0.96 seconds (consistent with my observation), and base duration is static.
Anyone seeing similar results?