It really depends. If you have a good, small enough team, and a clear design, with a well defined and limited scope, it shouldn't take that long.
If your team is too large, and especially if you don't know what the use case is, it can take a very long time. You asked for general purpose and fully capable, so you're probably in this case, but I think the desired use cases for Fuchsia could be scoped to way less than general purpose and fully capable: a ChromeOS replacement needs only to run Chrome (which isn't easy, but...), and an Android replacement needs only to run Android apps (again, not easy), and the embedded devices only run applications authored by Google with probably a much smaller scope.
But it also depends on what 'from scratch' means. Will you lean on existing development tools, hosted on an existing OS? Will you borrow libraries where the scope and license are appropriate? Are you going to build your own bootloader or use an existing one?
If your team is too large, and especially if you don't know what the use case is, it can take a very long time. You asked for general purpose and fully capable, so you're probably in this case, but I think the desired use cases for Fuchsia could be scoped to way less than general purpose and fully capable: a ChromeOS replacement needs only to run Chrome (which isn't easy, but...), and an Android replacement needs only to run Android apps (again, not easy), and the embedded devices only run applications authored by Google with probably a much smaller scope.
But it also depends on what 'from scratch' means. Will you lean on existing development tools, hosted on an existing OS? Will you borrow libraries where the scope and license are appropriate? Are you going to build your own bootloader or use an existing one?