After you have passed a vehicle moving in the same direction, it is safe to move back into the right lane:

Question

When passing, you should not return to your original lane until you can see both headlights of the passed vehicle in your rearview mirror. Always check your blind spots before moving back into your original lane.

Accepted Answer

When you can see the entire front of the passed vehicle in your rearview mirror.

Answer

After about three seconds.

Answer

When you can no longer see the passed vehicle over your right shoulder.