Stub zone refresh interval?

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.