Hi everyone,
I just "invented" a smart way, how to check how long the Sansa's battery can last, even though battery_bench doesn't work on the Sansa yet.
Simply play the root.m3u which you can generate by selecting "create playlist" from the Playlist Menu in the Main Menu. Make sure that you start at the first song and make sure the whole list is longer than the Sansa's battery life (in my case the whole list lasts about 3 days, so that's definitely more than the maximum battery life).
After the battery has run dry you can connect the Sansa to your PC, recharge the battery and then (hopefully) find out, at which song (i.e. at which position in the playlist) the playback stopped. Now you can calculate how long playback has been running.

[Does Rockbox save the playback position even when it is forcefully shut down because it was running out of battery? If it doesn't, then of course the trick won't work.]
I have yet to actually try it, and I know it's kind of stupid, but I can't think of a smarter way to probe the battery life without battery_bench.