Like above guy mentioned, avarage 2sec added into defenders attack cooldown.
(0,5sec + avarage 2sec, for water gun as example)
So vaporeons water gun attacks you every 2,5seconds.
Where confusion pokemon would attack you every 3,6seconds (1,6sec + avarage 2sec)
Exception: all defenders 2 first attacks are fast hits.
I believe 1sec between first and second hit.
So water gun + 1sec + water gun (0,5sec + 1sec + 0,5)
confusion + 1sec + confusion (1,6 + 1sec + 1,6sec)
and most likely they also wait 1sec before hitting you first time.
Anyways, the point: after 2 first hits, you add avarage 2sec into the attackers cooldown.
All moves have different cooldown.
Note: This is why confusion as defender move is much better than water gun
Tiny bit of additional detail on the first two quick hits by a defender. The first one hits with 98.4 seconds remaining and the second with 97.4 seconds, regardless of how fast/slow the quick attack actually is (so the animation will actually not complete for some quick attacks before the second attack starts).
(That's compared to the first of the attacker's hits, which comes with 99.3 seconds--0.9 seconds earlier)
This might be useful...