Akula,
If you go to Workshop settings, look under "Campaign Option" and go to "Campaign Date Options".
Check the manual option. After that a box with a "T" will come up when you go to your campaign mission screen. Press it. This will bring up your Advance Options. The "Request leave" only comes up after you have flown enough missions and I do not know what that time is off the top of my head.
Hope this helps!