Stub zone refresh interval?

[David Carmean]

For BIND-9.3.0, what sets the timing for the stub zone refresh 
checks?

I've set up a testbed, and with the following SOA on the master:

    example.com.            120 IN SOA int-ns17.example.com. dlc.example.com. (
                                18         ; serial
                                330        ; refresh (5 minutes 30 seconds)
                                900        ; retry (15 minutes)
                                1200       ; expire (20 minutes)
                                600        ; minimum (10 minutes)
                                )

whereas I expected it (the resolving server with the stub zone clause)
to see the new serial number and update after about 330 seconds, 
it actually took 28 minutes.