Welcome to Android Central! The OS usually takes about 3-4 GB, and then additional preinstalled apps ("bloatware") can take up more--anywhere for less than 1 GB to another 4 GB. Samsung has been notorious for piling on lots of preinstalled Samsung apps. The only way to fix that problem is to root the phone and uninstall some of the preinstalled bloat, but you have to be careful, since removing some apps might have an impact on the performance of others.