How long was a trip in the Alien franchise?

Question

In Alien Dallas says he's been on five trips with a different science officer who was replaced on this trip with Ash.

How long were Ripley and the Crew of the Nostromo expecting to be away from Earth?

Answer 1

The original trip was supposed to take "several" months in each direction, probably about 3-4. When the ship went off-course to check out the signal, it took an absolutely massive detour, placing them nearly ten months away from Earth.

Dallas wakes up thinking he's being awoken for final approach...

Oh well. After several months of doing nothing but sleeping, he had no right to complain if a few hours' honest sweat was now required of him . . .

_{^{Alien: Official Novelisation}}

The crew then find out how far off-course they've gone

LAMBERT: It'll take us six weeks to get back on course.

DALLAS: How far to Earth.

LAMBERT: Ten months.

RIPLEY: Christ.

^{_{Alien: Shooting Script}}

The implication being that this length of time is dramatically longer than they were expecting to be away from their home base and loved ones.