The campaign in The Ascent takes about 15 hours. However, discovering all of the game's content will take at least an extra few hours. After completing the last mission, you will see the end credits. This page answers whether you can keep playing the game after finishing it.
Can you play after you finish the game?
Completing the last mission called Unhandled Exception doesn't mean that you won't be able to go back to the game. You can continue playing after finishing the campaign. The player is moved to Cluster 13 - you can do side quests or continue to explore the world. You don't have to worry about losing some of the content after you finish the game - completing the main campaign doesn't affect the rest of the game in any way.