4 of 4 added 29 characters in body

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?