If possible, you must allow three-5h of transit time in Johannesburg, so that you've got sufficient time for you to capture your connecting flight. Transiting in Johannesburg is very easy; simply just Keep to the indicators for ‘International Transit’. The flight from Johannesburg to Harare usually takes about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/