The system takes 1.71 GB on the 920, so you're looking a little over 2.25 GB available if its the same on the 8S. It may be a little less.
And a little more perspective, just set up a Nokia 820, and it has system taking 1.7 GB, so it does seem they optimized it a little for phones with lower memory. Best advice, go to store that has one, go to settings, and go down to phone storage. Tap that, then the bar and it will tell you how much the system occupies.