What Seven of Nine is describing is not the time it'll take for them to get to Earth but the amount of time it'll take to cross a region of space that's devoid of stars (that they later dub "the void").
SEVEN: Then I must disobey. I have no good news to report. I've completed an astrometric scan of the entire region. There are no star
systems within twenty five hundred light years.
CHAKOTAY: Why can't we see stars beyond that?
Crossing this region represents approximately 3% of their total journey back to Earth. In the final analysis, they manage to locate a 'spacial vortex' (basically a wormhole by another name) which dramatically shortens their trip, enabling them to traverse the void in just a couple of months.
EMCK: There's a spatial vortex a few light years from here. It leads directly to the other side of the expanse. We've been using it
CHAKOTAY: That would get us out of the Void, and cut two years off our journey home.
The moderately canonical Star Trek: Star Charts gives us some idea of both the scale and location of 'The Void'