Every Alien movie in order
To watch the Alien movies in order, start with Prometheus, then Alien: Covenant and Alien. That's 7 out so far, about 14 hours, ending with Alien Resurrection.
The prequels, Prometheus and Alien: Covenant, come first in the story, and Alien: Romulus fits between Alien and Aliens. Ripley's four films run from Alien to Alien Resurrection.
Updated
The Alien movies in order
Prometheus
Alien: Covenant
Alien
Alien: Romulus
Aliens
Alien³
Alien Resurrection
Also worth watching
Alien: Earth
The series, set two years before Alien.
Questions
What order should I watch the Alien movies in?
Start with Prometheus, then Alien: Covenant and Alien, and follow the list above: 7 in all, ending with Alien Resurrection.
How long does it take to watch all of the Alien movies?
About 14 hours for the 7 out so far.
Should I watch the Alien movies in release order?
For a first watch, release order is best: Alien, Aliens, Alien 3 and Resurrection, then Prometheus, Covenant and Romulus. The prequels spoil less when you know the originals.
More to watch in order
Watch the Alien movies with Chronologica
Make a plan of just the Alien movies, mark them off as you go and always know what's next. No account needed.