Also, the most southern pier is 10 and 13th is the furthest away. I calculated it out on Google maps. In terms of distance, if you are pier 10 or 11, it's closer to walk directly instead of going to Largo della Pace.
So back to my original question... If I have to take the free shuttle, how long is the longest you think it would take to get on the shuttle? I know it's like a 15 min ride and then I'd walk to the train station which is another 20 min walk or so.