I don't actually keep any records on how long I play, I have started playing some days at 10.00 am. and gone all day only stopping to make a coffee or Sandwich, finishing the day with a Cardschat freeroll, which starts here in the UK. at around 8.00 pm.
In a Live Tournament I played in a Casino I started playing at 12.00 noon and finished on the Final table, taking 2nd place at around 9.00 pm. but the travelling time to and from the Casino added another 4 hours onto my day.
Keeping my mind on the game for so long is Mentally taxing but, being a Retired Taxi Driver Long hours have never been a problem for me, My longest unbroken day driving was 28 hours straight, Dangerous? Quite possibly, but not illegal. I have known other Drivers to do longer Stints.