Old question, but another insight I'd like to share

I assume people will want to charge the supercap when normal power supply is on, and that will bring diodes to the curcuit, (see image) reducing the effective stored energy your RTC can use, leaking significant amount nobody mentioned so far here.

[![Supercap charging with diodes][1]][1]

I found it on [particle.io](https://community.particle.io/t/photon-vbat-supercap-circuit/12390/) They also mention that STM micro don't tolerate sinking current back from VBAT input when: you start charging the cap that is below Vin-0.6V. Most of the discrete RTCC's are like that too, that's way you need D2. Reason D1 is pretty obvious, you want only the RTCC to use the supercap's stored energy.

Choosing Schottkys will be a tradeoff (yet again). The lower the forward voltage you choose, most probably the higher the reverse leakage current will be.
For example a BAS-40 (you can find two in series configuration in SOT-23, with "S" postfix) will have 0.4V voltage drop if you charge the cap with 10mA at 25C (see [datasheet](https://assets.nexperia.com/documents/data-sheet/BAS40_1PSXXSB4X_SER.pdf/)), AND will leak on the order of 0.1 uA at normal temperatures. If you choose another schottky, leakage can easily be ten-hundred times higher. That can be explain the value [Kasi](https://electronics.stackexchange.com/users/46509/kasi/) measured in the previous answer.


  [1]: https://i.sstatic.net/dRRe3.png