How did the time to Earth get shortened?

I am currently in Season 5 episode 1 of Star Trek: Voyager. Chakotay was discussing with Seven of Nine that they have been travelling for months through the expanse and that he doesn't know how they will last two more years of travelling.

I thought they were going to take like 50 years or so to get back to Earth. It seems I missed something out and I'm unable to connect the dots. Did they get a new engine or something? Did they again get thrown ahead in space (but that far?)

The question is: Which episode made them reduce their time to make it to Earth?

• what am i missing and what do i need to watch? – Tito Jan 20 '17 at 13:22
• This episode is called "Night" en.wikipedia.org/wiki/Night_(Star_Trek:_Voyager) – maguirenumber6 Jan 20 '17 at 13:32
• I think you misunderstood. The 2 years Chakotay doesn't think they'll be able to survive was not how long it takes to get to Earth, but the time to get through the void of "empty space" – starpilotsix Jan 20 '17 at 13:39

Just a quick read of the wikipedia article suggests you've misunderstood what they were talking about. The two years was how long it was going to take to cross the Void,

where starlight is completely shrouded, creating a dense black cloud. The journey through this sector will take two years, but only two months into the travel has set the crew on edge.

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: Nothing?

SEVEN: Nothing.

CHAKOTAY: Why can't we see stars beyond that?

Voyager: Night

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 for years.

CHAKOTAY: That would get us out of the Void, and cut two years off our journey home.

Voyager: Night

The moderately canonical Star Trek: Star Charts gives us some idea of both the scale and location of 'The Void'

Maybe they found that worm thingy and went in through that. That is the only way this could happen. And since Chakotay can get the help of his spirit beast.

Also, since Janeway knows Q. (That should actually be the answer to everything in this show.)

Janeway knows Q!