When passing on the left of a vehicle, it is safe to move back into the right lane:

When passing another vehicle on its left, you may move back into the right lane when the headlights of the passed vehicle can be seen in your rearview mirror. Always signal before changing lanes.

• After about three seconds.
• When you can no longer see the vehicle over your right shoulder.
• When you can see the front of the vehicle in your rearview mirror.