If possible, you'll want to let three-5h of transit time in Johannesburg, so that you have ample time for you to capture your connecting flight. Transiting in Johannesburg is rather easy; simply Keep to the indicators for ‘Intercontinental Transit’. The flight from Johannesburg to Harare usually takes about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/