I saw a small reduction in battery life with the upgrade to 10.8 but nothing like yours. Check your battery life - Look under the Apple menu for About this Mac, click on the More Info button and then the System Report button. On the left look for Power and select it. Here you can see how many cycles your battery has consumed, its fully charged power level and current power level. You can also see your battery's health. Your problem could be the battery itself.
Also run the Activity Monitor for a while and watch your CPU usage. Except for when a power hungry application is running - a music ripper or video renderer - your CPU usage may spike above 50% but should remain pretty much in the 20-40% range. If it is constantly above 40-50% look for the application(s) that are running hot - they could explain the battery loss. For example, shortly after I upgraded to Lion I not only had short battery life but a fan almost constantly going. I discovered that my computer was constantly indexing the drive. With that fixed my computer returned to normal.