I have being using my N900 with an original Nokia BL-5J battery for 11 months. I always let it run dry before recharging it to full capacity.

I haven't noticed any degradations yet (though they should be there). However, in my experience, buggy applications are what consume most battery.

Since installing Pr1.3 and also installing the hildon-desktop fix, my personal record is almost 6 days of low usage. Just after pr1.2 and before the hildon-desktop fix I could only get at most 2 days with low usage.

So my advice is just to check for stray applications, widgets or anything that is left eating your battery charge away.