The Colonial Marine's Tech Manual goes some way towards addressing this. Citing the Nostromo's speed capability as about...0.1 to 0.12
light years per sidereal day (40-45 cee
) while fully laden, and around 4 times faster when unladen, about... 0.42
lys per sidereal day (153 cee
), which is still a trip of about 3 mos, back in 2122, in a ship that's not really well maintained.
Flash forward 15 years to 2137. The Torrens departs Sol on Nov 15th and arrives at Sevastopol Station on Dec 11th, about 26 days later. This gives the Torrens a fast, but not emergency, speed of about... 1.5
lys/sidereal day (550 cee
). Which kinda makes sense, the Torrens is working as a courier, instead of a refinery hauler and is much better cared for than the Nostromo was, Verlaine has no doubt kept her maintained and upgraded with the latest engine tech.
Skip ahead about 42 more years to 2179. The heavily armored Sulaco has a fuel efficient high cruising speed of about... 0.74
light years per sidereal day (~271 cee
) . But, if needs be, it could haul ass on full burn and pull around...1.8
light years per sidereal day (~666 cee
) and get from Sol to Zeta 2 Reticuli in a breakneck 21 days.
The grandness of space is still there if one considers that the area of explored space is around 20 parsecs from Earth. The Milky Way is between 50,000 - 60,000 parsecs across and averages 300 parsecs thick.
True that, even in the late 2600's when the human sphere is nearly 1000 lys in diameter, that's still only about 1% of the galaxy.