Maximum productive time for a meditation sit?

I've been searching around forums for the maximum time that is productive to do a single meditation sit. There are lots of comments around over 20 minute mark for effectiveness and sitting for the hour is the most I've seen anyone say they sit. But what is the maximum that one should reasonably sit for before it becomes counter productive or at least diminishing returns set in? As a side note I remember Jack Kornfield relating that a retreatant of his declared that he was going to sit for as long as it took to get enlightened. He didn't get enlightened and the whole episode didn't go that well for that individual