Graf has 377 weeks at #1, Navratilova has 332. Serena has 123 weeks.
Another 200 weeks or so and she could be there depending upon what else she does.
That would be about 4 years more at #1. Given her age, it would be a very tough task. However, if she dedicated herself completely to it, she could get at least a couple of years of being #1. I wouldn't say it would be impossible for her to get 4 years.