If Possible, you ought to allow 3-5h of transit time in Johannesburg, so that you've got enough time and energy to capture your connecting flight. Transiting in Johannesburg is rather easy; only follow the signs for ‘Global Transit’. The flight from Johannesburg to Harare will take about 1h 30m. The https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/