I think I know what answer you are after.
Windows takes around 15-20GB of space, so if you have a 120GB SSD, you will have around 90-100GB of free space available for your games and programs.
It is saver to install Windows on it's own partition (e.g. 50GB or so) and keep all your data e.g. documents, pictures, music on a different partition or drive. So that when Windows fails/crashes you can just reload and all of your data should still eb save on the other partition/drive