Before letting know about focal length of plane mirrors we will know what are plane mirrors?
Plane mirrors – The images formed from a plane mirrors are the reflected images in their normal proportions but reversed from left to right. These are the most widely used mirrors.
The focal length of a plane mirror is infinity as the image can be formed at an infinite distance inside the mirror. A plane mirror can be recognised as a spherical mirror of infinite radius of curvature. Focal length is half of the radius of curvature and hence it is infinity.