The time you get from the battery is simple to figure out when the battery is new. Just look at the mAH and if you want twice the time, then get one double the size.
I picked up a replacement battery from a maker of such, but don't go with unknown. Here's why -> http://news.google.com/news?hl=en&edition=us&ie=UTF-8&newsclusterurl=http://www.telecomskorea.com/index.php%3Foption%3Dcontent%26task%3Dview%26id%3D405%26Itemid%3D2
Seems some batteries are causing problems.
I recently bought a Sony DCH-HC40. The original battery that it came with lasts only about an hour. So, I'm looking to buy a stronger battery for it. The batteries provided by the manufacturer are on the expensive side (about $130). Similar replacement batteries are half the cost. My question is whether it is advisable to use a replacement battery, instead of the one that Sony sells. Do they give similar performance, and do not cause any degradation to the camcorder ? Any help will be appreciated.