With a 8GB 2nd gen:
As little as four hours of using something like BBC iPlayer (because it uses most of the hardware- WiFi, CPU, video decoding, sound), or some intensive game with 3D graphics, the sort where you can feel the heat coming from the lower left corner of the device where the CPU is - a sure sign it is eating battery power. That's all with the screen brightness set quite low (less than 1/4 max) with auto adjust on in what is generally a quite dimly lit room (which makes the display even less bright). If the screen brightness were higher, battery life would be further shortened.
That's the worst case scenario though. I can generally get seven or so hours of "normal" usage, such as a bit of web browsing, a bit of reading news stories, a bit of document viewing, the odd undemanding game like Sudoku, and so on (with WiFi disabled except when I'm actively using it).
In extreme cases where it is quite literally "doing nothing" such as not having any app running (when I forgot to turn it off), or something totally undemanding like an alarm clock (I have my device set so the display never automatically turns off) I can get as much as about eleven or twelve hours before the battery finally runs out.
I think the 30 hours for music is a best case scenario where the display is off for pretty much the whole time and it is just playing through all the stored music at low volume into earphones.