I use Coconut battery but I don't think that's what you're looking for.
What you're looking for doesn't seem like a very practical app to be honest.
Opening and closing your computer, different day to day computing, etc. all will change the "total time" you get out of a charge so even if there was an app it wouldn't really tell you much. You could get 5hrs one day and get 7hours the next and 4hours the day after that...a few youtube videos and some downloading could change a lot.
Sorry mate