I was a child in one of those Turkish families going back to visit family during the summer. Almost every year from 1992 to 2007, we were taking a ferry from Brindisi/Bari to Igoumenitsa and then crossing the whole Greece on what was just a normal road, not yet the huge impressive motorway. We could see from the old road the huge pillars and tunnels being built. All that for nothing, because when the Egnatia Motorway was finished, there was no traffic on it anymore. In 2007, my last trip, the motorway was empty, and even the ferries from Italy to Greece were becoming rare: what killed it is in my opinion 2 distinct factors : there were no Turkish families anymore because plane became much more attractive (cheaper, easier, less tiring) ; and freight did not want to bother anymore driving along Greece, when the brand new UND Roros (now DFDS !) were linking everyday Trieste to Istanbul. (And from all these trips across the adriatic came my interest in ferries :-) )
Now to be back to the question initially asked of the longest ferry crossing, Maybe we can consider Hurtigruten in Norwegian Coast. From Bergen to Kirkenes, it seems to take about 6 days, with all the stops of course. Technically, even if they don't look like it, they do have a small car capacity !