You can not simply answer how long a battery will last, it's 'rated battery life', which you can probably find on the laptops model website. Is just a very breif guildline and not at all what you can expect sometimes.
The battery life can depend on
1. CPU clocks
2. what you're doing on your PC
3. screen brightness
4.temperature
So it's hard to say.
Look on the manufacturers website and take it with a grain of salt.
-good luck