For example I have 2000 mAh. How do I calculate how much longer my device will stay on?
It depends on the hardware of the laptop, what sort of software you're running, whether you're using the computer actively or if it's just sitting on the desk, what sort of power management is configured, the environment you'll be using the laptop in, and how old the battery is, just to name a few factors.
There is no way to accurately answer this question. Even if you were to provide extensive and accurate details regarding the above criteria there is still natural variation which would make any answer a guess.
If you need lots of battery life, buy a big battery and turn off every extraneous start up process on your computer. If you need more performance and the laptop will be on your desk most of the time, get a small cheap battery.
To find out how long it'll last, run it until it gets to very low power (10% or so) and time it.