Ideally, It is best to allow 3-5h of transit time in Johannesburg, so that you've sufficient time and energy to capture your connecting flight. Transiting in Johannesburg is quite quick; simply follow the indications for ‘Intercontinental Transit’. The flight from Johannesburg to Harare usually takes about 1h 30m. A similar https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/