Generally they are different each week but occasionally they last for longer. An example of a longer one would be where player factions had 4 weeks to deliver as many travel guides to Colonia as possible. Those in the top (something percent, I forget) had their faction put in Colonia.
Catch was, you could only buy 10 guides at a time so we had to work together to help fill the delivery boys/girls. I spent many an hour going from station to 10km giving Mobius Commanders travel guides but the real hero's were Andy Bulman, who used an anaconda as a sort of warehouse and Duck of Death who did A LOT of trips to Colonia