I agree with the other people comments, it does depend on what you are using it for. If you was buying a new laptop now then 16GB is standard.
I remember going from 4GB to 8GB and the difference is most notable, as when you ran out of 4GB memory it paged it out to your hard drive hence feeling slow. When I went from 8GB to 16GB it is less notable but does depend on what programs you are using.
If you go to the Task Manager, click on Performance tab, then Memory, it should give you an idea of your current usage and what memory is left available.
Also remember that if your Laptop uses onboard graphics, then some of your memory will also be used up for this purpose.