Ideally, You'll want to make it possible for three-5h of transit time in Johannesburg, so that you've enough time for you to catch your connecting flight. Transiting in Johannesburg is quite quick; basically Keep to the indicators for ‘International Transit’. The flight from Johannesburg to Harare usually takes about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/