I got my Quicksands caves map, I picked a down time when there wasn't too many people. Also I choose my 39 WHM because I'm not lving it at the moment and if I died, which I did, I wouldn't loose xp from the job I am leveling (RDM). I found the coffer on the 2nd time around. Mine was at J-13 on the back wall of the sand part in the room. My higher level friends tell me that once a coffer is opened that it can take anywhere from 15-30 minutes to respawn and uncomfirmed but they also say that the coffer can change points every 15 minutes if it's not opened.
Happy coffer hunting ^^