If possible, you should allow three-5h of transit time in Johannesburg, so that you've got ample time for you to capture your connecting flight. Transiting in Johannesburg is quite easy; only Stick to the signals for ‘Global Transit’. The flight from Johannesburg to Harare normally takes about 1h 30m. Nationwide https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/