Theoretical backup time can be easily calculated, but leakage current is not necessarily specified.
In practice, small super caps, e.g. Panasonic Gold Caps are only good for one or several days backup time. That may be O.K. for devices with permanent mains connection to bypass power fails, but not for portable devices. A usual requirement is also that devices don't loose there date and time information between production and commissioning. That's only achievable with long life batteries.