I am currently in Season 5 episode 1. Chakotay was discussing with 7 of 9 (Borg Lady) that they have been travelling for months through the expanse and that he doesn't know how they will last 2 years 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?**