Setting up Unity to create Quest content is actually pretty straightforward. This image covers it pretty well:
You need to install Unity's Android SDK. Otherwise the option won't pop up.
If you're starting a brand new project, this won't take long at all. However, if you're converting a Windows platform project to an Android platform project, you will have to convert your assets appropriately. This can take quite a while for larger projects.
For more details on best practices when working with dual-platform projects, read our documentation on Cross-Platform Setup.