It can get up to 20h either when idle with all hw switches off or when suspended with the modem on (less with poor signal coverage, of course). Looking at a power meter right now I'd roughly expect at least 12h of music playback.
Check whether you get a high load average whenever Wi-Fi kill switch is off. If so, keeping it on and turning Wi-Fi off in software may give you better results. There are patches to fix that, but they need more work not to cause troubles with older cards (Redpine).
There was also another issue that may have caused interrupt storms when kill switches were set to off, but that has been already fixed recently.
There are some small power consumption improvements coming in kernel updates soon too, though these shouldn't make a drastic difference, just an extra hour or so.
Using the latest greatest stock pureOS?
My L5 gets nowhere near that listening to MP3s on road trips.