Both 25th Anniversary and Judgment Rites are traditional adventure games in the Sierra vein, so there's a lot of dialogue and puzzle-solving, all of which is linear. There is scorekeeping so you can always try again and see if a different approach gets a better score (hint: don't vaporize things with your phaser unless you can help it.), but otherwise that's it for replayability.

While it's theoretically possible to beat 25th Anniversary in 2.5 hours, that's almost certainly with a walkthrough, or with genius-level intelligence. It's broken up into 7 episodes, and each one will probably take about 30-60 minutes to play through.

Judgment Rites is the sequel. It is a better game because they were able to refine on what they did before, but it picks the plot up from 25th Anniversary immediately so I wouldn't play it first.

Starfleet Academy is completely different and is mostly a flight sim with interactive story sections that take place between missions. It's fun, but I'd argue that 25th Anniversary and Judgment Rites are better games.