If possible, it is best to let three-5h of transit time in Johannesburg, so that you've got sufficient time to capture your connecting flight. Transiting in Johannesburg is quite simple; simply just Keep to the indicators for ‘Intercontinental Transit’. The flight from Johannesburg to Harare normally takes about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/