Preferably, you'll want to enable 3-5h of transit time in Johannesburg, so that you've got sufficient time and energy to capture your connecting flight. Transiting in Johannesburg is very easy; simply just Stick to the signs for ‘Global Transit’. The flight from Johannesburg to Harare will take about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/