It's policy to give no rostered days off above the minimum legal required - i.e. 8 or 9 per month, depending on number of days in the month.
Any more than that is stby duty.
Having said that, it's about a 50/50 chance on the 777 / A330 of actually getting through your stby without being called out, whereas you're virtually guaranteed to get called on the A320 fleet
