Your mileage may vary...I get about 2hours 45minutes on the default settings...more if I reduce the display lighting.
As for charging of lithium ion batteries, the biggest enemy is heat. Keeping the battery charged up unless running cordless would be the least damaging as the battery does not heat up unless under charge or discharge. Unlike the older NiCd or NiMH batteries (less so for the NiMH), you do not need to discharge the battery periodically. There are differences in the cells, depending upon manufacturer. The OEM battery (at least the ones I have seen), are Sanyo, which are high quality cells. Some of the ebay specials have questionable construction and cells, so you may find lower battery life. Also, I have found two different ratings on the 5670's I have seen, either a 4400mAH or a 4800mAH rating (mine is the latter).
Oh yes, for those who feel their track pad is too hot, I have tried the Targus brand cooler (with USB hub). Works well, and keeps the temperature down to under 85F even when using full power (actually, the trackpad area seems to be running about 8 degrees above ambient with the cooler, 10 to 12 w/o).
Acer Aspire 5672 Intel Core Duo 1.66, 2G ram, x1400, 120GHD