The length of the average intro to a song can vary widely depending on the genre, style, and era of the music. Generally, intros can be anywhere from a few seconds to over a minute in length.

In popular music, intros tend to be shorter, often ranging from a few seconds to around 20 seconds in length. Some genres, such as electronic dance music, may have even shorter intros, often around 8-16 bars in length, to allow for easy mixing and blending of tracks by DJs.

On the other hand, some classical music pieces may have longer intros, often several minutes in length, as the intro may serve as an overture or an introduction to the main themes of the piece.

The length of the intro to a song can vary widely and is largely determined by the artistic choices of the composer or songwriter. Some songs may have no intro at all, while others may have an extended intro that sets the mood for the entire piece. The most important thing is that the intro serves its intended purpose of engaging the listener and setting the tone for the rest of the song.