Makenshi42
I've heard people say that you shouldn't ever play FFXI for less than 2-3 hours at a time, which is a problem for me, because I can only ever squeak in 2 hours if I'm lucky, its normally only 1 hour.
sweatdrop Its true that a higher lvl party will expect you to stay for at least 2 to 3 hours, and if you have to go you'll need to find yourself a replacement and wait for them to arrive. However in the dunes all deals are pretty much off. People behave like asshats in Vakrum xD
I would definitely advise you to put your lfp flag up while in Konstatch and see if anyone /tells you with an invite, even if you only have one hour to play.
However for curtosey sake you should inform the leader before you accept the invitation that you can only play for this limited time. Its also polite to give half an hours warning before you have to leave the party, and find a replacement for yourself if you possibly can.
Party turnaround in the dunes is relatively fast due to the close proximaty of all the lfp people (i.e. if you have to leave it doesn't usually take long to find someone and get them to camp).
After the dunes, parties take more effort to construct and commonly set up in Jeuno or Whitegate before heading out to a desired camp together, which may be far far away. It is in these instances where it is inpolite to leave too early.