About 35 GB. There are some space saving methods that you can use:
1. 4GB: Disabling hibernation. This will essentially make the SP3 behave like the Surface RT in standby, it goes to Connected Standby forever.
2. 1-2GB: Reducing pagefile initial size. Let the maximum size be large so that it can expand when needed. At least you can reclaim the space with a restart.
3. 5GB: Moving the recovery partition to a USB stick.
4. 10GB: WIMBoot setup. Most Windows files will be linked to the compressed recovery partition, hence not compatible with method #3. There is practically no performance or battery life difference compared to the traditional method. Check my sig for more information about WIMBoot, or the i3 SP3
Yeah, note that 35GB figure is before the 6+ gigs for hibernation and page file. When I first tried the 64 gig model I installed only Office including the last 12 months of mail in Outlook and two gigs of OneDrive data. Literally nothing else. By the time I had installed all the updates I was at 15GB free. That was cutting it too close for me, so it went back on the second day of ownership.
With a monthly cleanup of not needed files and updates, mine is steady at 24 GB, no tricks, recovery and so on are still there. Installed Office 365, use Onedrive and about 35 apps from store.
My Documents/downloads and so on are on my microsdxc