On my Ubuntu machine, I rarely use more than 1GB (apart from the disk cache, of course, which can fill all remaining memory. Note: windows only has an 8MB cache)
Of course on Microsoft Windows it uses all available memory, quite effectively. If you have copious RAM you get far-better-than-SSD "read speeds" when reopening applications (which is a typical behavior, cycling between a small universe of applications), presuming your computing lifestyle is that your computer is never turned off...e.g. you put it to S3 sleep during off time.