Also depends on what you are doing with it at the time. Working with things like high resolution scanned documents can take a lot, while just surfing the web and checking email takes much less.
When current usage starts getting close to to what you have for physical memory, the system will start trying to move less used sections to the hard drive. Since access time for the hard drive can be 100,000 times longer or more, it doesn't take very many misses for things to slow way down.
If you are looking for more performance, look into replacing the hard drive with SSD. They fit the same, so any laptop can make the change, and the performance difference is substantial.