It varies widely based on multiple factors.
- Just playing songs uses less charge than playing videos (video podcasts, movies, TV shows).
- Using a different brand of earphones might the energy used
- The sound level you set for the earphones
- The age of iPod with respect to the number of charge/discharge cycles.
From my experience on an hand-me-down iPod nano 5G I use, you get several hours of song play at sound levels that don't lead to premature hearing loss, and about half that time for pure video play (movies, podcasts).