I agree with this. I did three co-op terms (I'm not sure how they're formally different from internships but anyways...) two of which were four months long, and one which was eight months long.

I found that after three months, I was just getting to the point where I could make meaningful contributions. For my two four-month co-ops, this meant that I hardly got to contribute. I really preferred the eight month co-op term, where I felt like I was much more part of the team.