If you're on the skills list and have the necessary points, IELTS (if applicable) and a positive skills assessment, you can register an EOI for a 189 whenever you want - onshore or offshore.
With regard to the two years, I think you're getting confused. On a 457, an employer can sponsor you for permanent residency after two years of work. Whereas a 189 visa is a permanent residency visa, so you would have that status as soon as it is granted.
After living in Australia as a permanent resident for four years, you can apply for citizenship. That might be what you're thinking about.