Say I had a Mega Heracross use Rock Blast on Togekiss. For the sake of experimentation, let's say that hit #3 took Togekiss down to 45%, which is below when Sitrus Berry activates.
Would Togekiss eat her Sitrus Berry between hit #3 and hit #4 of Rock Blast, or would it wait until all five hits had landed before eating the Berry?
And I know, technically five hits would KO Toge, but it's just an example.