Ideally, You should make it possible for three-5h of transit time in Johannesburg, so that you've enough time for you to capture your connecting flight. Transiting in Johannesburg is very effortless; simply Keep to the signs for ‘Worldwide Transit’. The flight from Johannesburg to Harare will take about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/