Ideally, It is best to permit 3-5h of transit time in Johannesburg, so that you've sufficient time and energy to capture your connecting flight. Transiting in Johannesburg is very easy; just Keep to the symptoms for ‘Intercontinental Transit’. The flight from Johannesburg to Harare usually takes about 1h 30m. KAYAK’s https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/