You ought to need a key at some point. Insider Preview just means you're getting updates more frequently (and before they're ready to be released to the general public). As for how long you can function without one, I'm not sure. But the one time I tried Windows for ARM64 on an M1 Mac via Parallels, I needed a key. Just find a Windows 7 Product key sticker from that old crappy tower you probably haven't thrown out yet. 😉