All songs exhibit patterns that are easily picked up by the listener.Rhythmic patterns are probably the first ones to be noticed, and might arguably be the most important ones in certain genres. A more subtle pattern is one that we call harmonic rhythm.
Harmonic rhythm is, simply put, the frequency of chord changes, and there should be a noticeable regularity. In general, songs will change chords every two, four or eight beats. In every song there will be changes to the harmonic rhythm, and that’s to be expected; too much regularity in any one element of a song can start to become a bit boring. It’s important, however, that the listener perceive a regular pulse – a pattern – in your chord changes.
You can use the harmonic rhythm to help convey the mood your song is trying to elicit. A slow harmonic rhythm will make your song sound relaxed and thoughtful, while chords changing on every beat considerably increases the energy. If you are trying to increase the overall energy of your song, you may achieve it by increasing the frequency of your chord changes.
Also, an interesting technique to experiment with might be to purposely increase or intensify the harmonic rhythm as your song progresses. Keep in mind, however, that increased energy is difficult to reverse, so you’ll likely want to use this effect in a final chorus, or perhaps the bridge.
-Gary Ewer, from “The Essential Secrets of Songwriting” website:
Songwriting tips Write Better Chords, Melodies and Hooks.