Ideally, You should make it possible for 3-5h of transit time in Johannesburg, so that you've got enough time and energy to capture your connecting flight. Transiting in Johannesburg is rather easy; merely Keep to the symptoms for ‘Intercontinental Transit’. The flight from Johannesburg to Harare usually takes about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/