Ideally, you should make it possible for 3-5h of transit time in Johannesburg, so that you've enough time for you to capture your connecting flight. Transiting in Johannesburg is rather uncomplicated; basically follow the symptoms for ‘International Transit’. The flight from Johannesburg to Harare usually takes about 1h 30m. The https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/