As frequent as any other banking service globally, i would expect its targeted less than say the UK or US because of the likelihood for better returns by targeting richer nations (this is pure speculation though)
The reality is this though: banks will not want to reveal if they have faced or are facing regular cyber attacks - its just bad publicity. theres no sure way of knowing how frequent threats are occurring whether that be in Zimbabwe or any other country of the world
i would argue and say they are facing threats every day. now before anyone gets paranoid and thinks the apocalypse has arrived, Internet sites face threats daily if not hourly, just because there is a "threat" it doesn't mean that the banking sites are necessarily vulnerable to that threat and/or being attacked based on that threat.