If possible, you need to allow for 3-5h of transit time in Johannesburg, so that you've got ample time and energy to catch your connecting flight. Transiting in Johannesburg is rather quick; simply just Keep to the indications for ‘Worldwide Transit’. The flight from Johannesburg to Harare requires about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/