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?
Yeah, it's brand new as far as you would consider in practice (they use existing libraries and the like).
The answer is not much time. The real question is how long to develop good quality drivers for a give platform (say, an x64 laptop)? How long to port/develop applications so that the OS is useful? How long to convince OEMs, app developers and such folks to start using your brand new OS? It's a bootstrap problem.
That would be surprising. Where do you get that? I don't mean toy OSes or experiments. Linux, MacOS and Windows are still in development and I can't imagine the number of hours invested.
IIRC it didn't take that long to develop first production versions of macOS? A couple of years maybe?
It's not like Fuschia was supposed to be a "fully capable OS developed from scratch", either? I mean it's "just" the kernel and other low level components, most of the software stack would remain same as Android/Linux at least for the time being.
> first production versions of macOS? A couple of years maybe?
Ok, I'll bite. If we're talking classic Macintosh OS, perhaps.[0] macOS? No way. The first Mac OS X was released in 2001, and was in development between 1997 and 2001 according to Wikipedia.[1] But the bulk of the OS already existed 1997. Mac OS X was a reskin of NeXTStep. NeXTStep was released in 1989, final release 1995, final preview 1997 (just before Apple sold out to NeXT).[2] NeXTStep was in production for quite some time before the x86 version shipped (around '95 from memory). In case you are wondering, I can assure you that NeXTStep was a very capable OS. NeXTStep was in development for a couple of years before the first hardware shipped in 1989. NeXTStep was built on top of Mach and BSD 4.3 userspace. Mach's initial release was 1985.[3]. Not sure how long the first release of Mach took to develop. You can check BSD history yourself. But I'd say, conservatively, that macOS took at least 14 years to develop.
> IIRC it didn't take that long to develop first production versions of macOS?
If you mean the early 1980s OS, that is not comparable. It probably ran in something like 512K of memory off of a 5.25" floppy disk (or a tape?).
> It's not like Fuschia was supposed to be a "fully capable OS developed from scratch", either? I mean it's "just" the kernel and other low level components
I don't know the answer, but doesn't the second sentence describe Linux?
The original Mac has 128kB RAM, a 64kB ROM with a fair chunk of the OS in it, and used 400kb single sided 3.5" discs. The paltry RAM is generally considered to be the main problem, but the Mac team were working to a target price of $1500 (which they missed), and that’s all that could afford, with the largish ROM being a compensation. A quick unscientific look at Byte's January 1984 issue seems to show 128kB as the base level for IBM PC clones at the time as well, but they don't have a GUI.
In comparison, the Lisa OS required 1MB RAM and a 5MB hard disc, hence the eye watering $10,000 introductory price.
Development on the Mac apparently started in 1979, and release in 1984 although the early Jeff Raskin era machine was quite different to the final Steve Jobs led product.
(IIUC, it's brand new?)