Ideally, You'll want to let three-5h of transit time in Johannesburg, so that you've sufficient time to catch your connecting flight. Transiting in Johannesburg is quite quick; basically Adhere to the indicators for ‘Global Transit’. The flight from Johannesburg to Harare requires about 1h 30m. The flights available are unrealistic https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/