Preferably, you ought to make it possible for three-5h of transit time in Johannesburg, so that you've ample time and energy to catch your connecting flight. Transiting in Johannesburg is quite simple; simply follow the indicators for ‘Worldwide Transit’. The flight from Johannesburg to Harare requires about 1h 30m. KAYAK’s https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/