If Possible, you ought to let 3-5h of transit time in Johannesburg, so that you have enough the perfect time to catch your connecting flight. Transiting in Johannesburg is rather easy; simply Adhere 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/