Original 630 mAh 8 hours use (Never Happened). The 8 hour rating was only on standby mode. I don't recall ever getting more than 6 hrs, but that dropped off to 4 hours pretty quick and held steady at that for a while. I generally used it on the bus ride to work, and always had to be sure to plug it in the night before. I also plugged it in while at work.

What is it down to? I expect it is good for about 2 hrs 30 min to 3 hrs.

If you get a replacement battery look for 850 mAh Extended life battery for 3rd Generation "3G" iPod's.

Here are some links:
http://www.sonnettech.com/product/ipod/batteries/index.html

http://www.ipodjuice.com/index.html#3rd_Generation_iPod_Battery_Replacement_Kits