If Possible, you ought to allow three-5h of transit time in Johannesburg, so that you have enough time for you to catch your connecting flight. Transiting in Johannesburg is rather quick; basically follow the symptoms for ‘Intercontinental Transit’. The flight from Johannesburg to Harare will take about 1h 30m. Delayed https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/