Ideally, You should permit three-5h of transit time in Johannesburg, so that you've enough time and energy to catch your connecting flight. Transiting in Johannesburg is very straightforward; only Stick to the indications for ‘Global Transit’. The flight from Johannesburg to Harare can take about 1h 30m. Fares are matter https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/