The rate at which a laptop battery discharges can vary enormously. For example, if I use my netbook with the lowest acceptable screen brightness setting, I'll get about 8 hours of light use (word processing, etc) from it. If I turn the screen brightness up to the highest usable level, that will come down to just two or three hours. Further, if I'm doing something which requires a lot of processing power (such as watching HD videos), the battery will run down much faster.
When looking for a new laptop you need to look at the 'mAh' rating of the battery and to read the reviews of the websites of technical magazines (etc).
Once you've got a laptop, you can get an estimate of how long the battery will last by going to the power icon at the bottom right of the screen. (You might need to go via 'Show hidden icons' to find it).