Ideally, you need to let 3-5h of transit time in Johannesburg, so that you have enough time and energy to catch your connecting flight. Transiting in Johannesburg is quite straightforward; merely Keep to the indicators for ‘Global Transit’. The flight from Johannesburg to Harare will take about 1h 30m. Overview https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/