Will it fill up my C: drive?

No. Two Left Wheels 360 never stores a stitched copy of your recordings. The large files on the virtual drive take no disk space: each frame is stitched from the original when your editor reads it. A stitched 8K copy would take about 20 GB per minute.

What it does keep, in the app's data folder in your Windows user profile, is small:

• Stitched proxy files, which make editing faster: about 8 GB per hour of footage, roughly twice the size of the camera's own proxy files. None is made when the disk has less than 20 GB free, and you can turn them off in the editing pack's settings.
• For a recording that came without the camera's proxy file, a proxy made for it, about the same size as the camera's own.
• Prepared audio, small index files and your settings and editing packs.

Files that no editing pack needs any more are removed automatically, and when you delete an editing pack you choose whether its files go too. Your exports are saved in the folder you choose. See How it works, in detail.