Hi,
On the 146, airbus or 737 you may get a direct command. What I heard about the 744, I think you have to get a little bit of experience on the right seat.
The rostering system is the same on all fleets . The latest to ask for a day off is 1,5 months in advance !! If within that period, you may change duty with a colleague if rest period permits.
Happy landings,