Ideally, You'll want to let 3-5h of transit time in Johannesburg, so that you have ample time to catch your connecting flight. Transiting in Johannesburg is rather straightforward; basically Adhere to the indicators for ‘Worldwide Transit’. The flight from Johannesburg to Harare usually takes about 1h 30m. Nationwide Artwork Gallery: https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/