Ideally, it is best to allow for 3-5h of transit time in Johannesburg, so that you have enough time to capture your connecting flight. Transiting in Johannesburg is rather easy; simply just Keep to the signals for ‘Global Transit’. The flight from Johannesburg to Harare normally takes about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/