On the 777 the average is around 15 days of work. A lot of flights arrive back to Dubai at 30 minutes after midnight so it is up to you if you want to consider that a day of work.
The reason for the late ( or is that early ? ) departures is twofold. Number 1 is that this is a desert and it gets hot in the desert. When the temperature starts climbing above 34 aircraft performance ( i.e. payload capability ) starts dropping. So afternoon departures can be somewhat limiting to revenue potential. Second is the way the connections work to and from Europe and Asia. One could go into great detail of how a passenger gets from Europe to Asia, but suffice it to say that it ties in with reason number 1 above.
Typhoonpilot