Preferably, you'll want to make it possible for 3-5h of transit time in Johannesburg, so you have enough time for you to catch your connecting flight. Transiting in Johannesburg is very easy; simply Adhere to the signals for ‘Intercontinental Transit’. The flight from Johannesburg to Harare can take about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/