According to the script for Back in the Red, Part I, the Red Dwarf is 3 million light years from Earth. I think we can assume this number has been rounded up or down slightly.
CMO: What's this rumour that we're three million light years into Deep Space, and Red Dwarf's changed shape?
HOLLISTER: That is classified information, Karen! Who the hell told you that?
Presumably the ship was traveling at a very appreciable fraction of lightspeed for the majority of its outward journey, then continued at an unspecified speed above lightspeed for several hours before finally coming to a halt. Their return journey was at a much lower speed, allowing them to stop the ship for fun and frolics.