Annual leave tracking (and setting your allowance) are part of the Pro Pack so you need to make sure you've subscribed to Pro first.
To set up your allowance:
Tap on the cog icon
Tap on Annual Leave Settings
This screen has 2 purposes; it contains the settings for your leave, how Time Off In Lieu (TOIL) is handled, and options for how leave is calculated. This screen also provides a report of time taken, leave remaining and other useful information.
Note: the Work Time Report also shows leave information alongside shifts worked and overtime, so if you haven’t already taken a look at this, it’s recommended.
The Work Time Report is available from the calendar menu by tapping on the calendar icon on the main screen.