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
I find daily sits of an unbroken hour and a half to be optimum.  The first half hour is normally just enough to settle my mind and loosen the hold that the day has on me.  The next half hour is spent really honing in on my concentration object (i.e. cultivating sustained thought).  It's here that my mind begins to unify into something resembling one-pointedness.  The next half hour is where the magic happens, so to speak.  During that part of the sit - and really depending on the day - is when my body starts to drop away and rapture begins to arise.  

Some days I may push toward two to three hours, but unless you are in a retreat situation, those longer sits don't necessarily get much deeper.  Your karmic obstacles simply have too much of a hold on your mind.  Only extended periods of silence and multiple 8+ hour days of meditation will allow you to progress into jhana.  

Side note: my teacher's teacher knew a monk who would just randomly sit for two days.  48 hours.  No sleep, no food, no bathroom.  In full lotus.   I can't even imagine doing that!