I tested time for listening with Nokia Lumia 800 & Spotify Mobile. Here's my results.
On the go with 100% power and Bluetooth mode on (I use Bluetooth headphones). Screen off, phone is used with basics on test. Only checked some settings and restarted Spotify some time, you know about this Bluetooth bug...
Listen with clean cache, power save mode on, 6 hours.
Listen with clean cache, power save mode off, 5 hours.
Listen with all songs in cache/offline mode, power save mode on, 12 hours.
Listen with all songs in cache/offline mode, power save mode off, 10 hours.
I listened tracks via 3G (0.5M, Unlimited). In my opinion this is good time, but if you heavy music listener like me, it's not enough when you want to be connected to the world (if anyone call to you, you want to check your mail etc.).
I suggest Offline mode for everyone, but in my playlist this is impossible. These days more than 3,945 songs.
Keep adding yours listening time with Windows Phone. I hope that this helps someone.