Ideally, It is best to allow for 3-5h of transit time in Johannesburg, so that you have sufficient time for you to capture your connecting flight. Transiting in Johannesburg is rather simple; only Keep to the symptoms for ‘Intercontinental Transit’. The flight from Johannesburg to Harare will take about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/