I have a 120GB as my main drive.
OS and all applications. Win 8.1 Pro, MS Office Pro, Adobe Lightroom, Paintshop Pro, Corel Video Studio, etc, etc, etc, etc
Games live on other drives.
Currently ~70GB used space.
It all depends on what applications you have. Install until you get to around the 80GB used level. This leaves room for future Windows updates.