If possible, you'll want to allow 3-5h of transit time in Johannesburg, so that you've enough time and energy to catch your connecting flight. Transiting in Johannesburg is very quick; simply just Adhere to the symptoms for ‘Worldwide Transit’. The flight from Johannesburg to Harare will take about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/