Preferably, it is best to make it possible for three-5h of transit time in Johannesburg, so that you've sufficient time and energy to capture your connecting flight. Transiting in Johannesburg is very effortless; simply follow the signs for ‘International Transit’. The flight from Johannesburg to Harare usually takes about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/