Ideally, you ought to make it possible for 3-5h of transit time in Johannesburg, so you have ample time and energy to catch your connecting flight. Transiting in Johannesburg is quite quick; basically follow the indications for ‘Intercontinental Transit’. The flight from Johannesburg to Harare requires about 1h 30m. KAYAK https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/