Ideally, You must let 3-5h of transit time in Johannesburg, so that you've got sufficient time and energy to catch your connecting flight. Transiting in Johannesburg is very effortless; only Keep to the indications for ‘Worldwide Transit’. The flight from Johannesburg to Harare will take about 1h 30m. Flights from https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/