We ended up with a good amount of free time during pre-fall last summer, but it was unpredictable. We fit 60 cadets on the ship during that. Not sure how many we just had on the first summer cruise between the GLMA and Texas Maritime cadets. It wasn't in rotations though. There are two seven week cruises this summer. I imagine next summer will be similar. We just got off the boat yesterday for the end of the first cruise.
As far as winter relief, I wouldn't bet on it at all unless you have prior deckhand experience. Before you're really even trained for the job, you have to go back to school, and the shipping companies just aren't interested in that.
Feel free to PM me with any questions. I know when I was coming in, I wished I had a cadet to bounce questions off of.