How long do you reckon the current vision of the game has been in development? 2-3 years?
Sounds about right.
Because I know when they first started their kickstarter it didn't have even close to the scope that it has now. It could take 4 more years to fully build this bitch out with all of the things they're trying to do. But if they succeed you'll never want to play any other game.
Honestly, it's very difficult to predict future development time because the kind of development remaining after 3.0 gets released is very different from the kind of development done so far.
The reason 3.0 is taking so long is that they are developing the groundwork that will be required to introduce features more rapidly later. An example is their internal tool for creating star systems. Just a couple of years ago they had to position everything manually within CryEngine, which really wasn't designed to handle maps larger than a planet. Now they've done the work to make that easy for designers to do rapidly with a simple UI that automatically handles things like orbital mechanics, using the Stanton system (the only one now playable) as a testbed, which means that future star systems will be done much more quickly than this first one.
Ultimately, it doesn't matter so much how long it takes to create the final game, as they've announced their intention to deliver more regular updates to the alpha after the 3.0 release. I think of it more as something that will gradually improve until it has all the features they want, rather than something that I'm waiting for the completion of. Because you're right, that could take years, but in the meantime we have a playable alpha that will continue to get better with time.