Preferably, you should make it possible for 3-5h of transit time in Johannesburg, so that you've got ample the perfect time to catch your connecting flight. Transiting in Johannesburg is quite straightforward; simply just follow the signals for ‘Worldwide Transit’. The flight from Johannesburg to Harare usually takes about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/