Preferably, you must make it possible for 3-5h of transit time in Johannesburg, so that you've got ample time and energy to catch your connecting flight. Transiting in Johannesburg is very easy; only Stick to the indicators for ‘Worldwide Transit’. The flight from Johannesburg to Harare normally takes about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/