Ideally, You should make it possible for 3-5h of transit time in Johannesburg, so that you've got enough time for you to capture your connecting flight. Transiting in Johannesburg is very effortless; merely Keep to the signs for ‘Global Transit’. The flight from Johannesburg to Harare requires about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/