On average, 5 to 15 seconds of music plays between each message blurb. Keep your audience in mind: musical breaks that are too long are boring, but message blurbs that are long or too detailed can be overwhelming. In most cases, 5 to 15 seconds, between message blurbs that don’t exceed 30 seconds are usually appropriate.
Finally, take into account the reduction in quality that goes hand in hand with hold music. The audio quality is—at best—24 times worse than that from a CD. Yes, we just said CD. The public telephone network wasn’t built with HD music in mind. Thankfully for OnSIP customers, we provide HD radio for VoIP music on hold. That said, your company might have top of the line audio quality, but if clients call in from an old landline or cell, they might hear some distortion.

For those still using analogue or digital phone systems, voice on ISDN circuits has a bandwidth of around 4000Hz. The audio from an analogue phone is sampled at 8000Hz, and because of the nyquist theorem, must be low-pass filtered to 4000Hz. So any frequencies in the audio beyond 4000Hz are lost. Interestingly, the human ear and mind are good at recognising human speech, so this is considered "toll-quality" voice, i.e. good enough to pay for. This is the reason why certain vocalised letters are difficult or impossible to tell apart on the phone. For example, the differences between B, P, T, C, etc. are almost entirely above 4000Hz.
The following list is my own personal “Top 10” list of songs which will not shorten your hold time, but may add a little levity to it: