If Possible, you should allow three-5h of transit time in Johannesburg, so that you've enough time and energy to catch your connecting flight. Transiting in Johannesburg is quite quick; just Adhere to the indicators for ‘International Transit’. The flight from Johannesburg to Harare normally takes about 1h 30m. KAYAK’s flight https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/