Rear-view mirror
When passing, if you can see the entire front of the truck in your rear-view mirror, it's safe to merge back into the lane.
Passing trucks
When passing a truck, allow one car length for every 16km/h per hour. For example, if you're travelling 60km/h leave four car lengths between you and the front of the truck before merging back over.
Maintain your speed
Once you pass a truck and return to the right lane, do not slow down. Maintain your speed and make sure there is a safe space between the truck and your car. Trucks take longer to regain their speed after being slowed down and may slow the traffic behind them.
Infographics from Barloworld Transport
Also view
Safely Sharing the Roads With Trucks - Trucks Make Wide Turns
Safely Sharing the Roads With Trucks - Avoiding Blind Spots And No-Go Zones
Safely Sharing the Roads With Trucks - Trucks Can't Stop Like Cars