Ideally, you must let 3-5h of transit time in Johannesburg, so you have enough time and energy to catch your connecting flight. Transiting in Johannesburg is quite effortless; just follow the indicators for ‘Global Transit’. The flight from Johannesburg to Harare normally takes about 1h 30m. The flights available are https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/