Preferably, you'll want to make it possible for 3-5h of transit time in Johannesburg, so that you have enough time and energy to catch your connecting flight. Transiting in Johannesburg is very effortless; merely follow the signals for ‘International Transit’. The flight from Johannesburg to Harare requires about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/