Ideally, you should make it possible for 3-5h of transit time in Johannesburg, so that you've got enough time and energy to capture your connecting flight. Transiting in Johannesburg is rather uncomplicated; just Adhere to the indications for ‘International Transit’. The flight from Johannesburg to Harare normally takes about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/