If you want to know how long Dead Space is then that depends on how much of it you want to see. The new remake adds in extra side mission elements that are completely optional but could add a few hours to your playtime. Especially considering how your access to the Ishimura depends on story beats opening up locked doors and letting you reach certain areas - its possible to look into side objectives you can't complete yet, which just adds extra time to your playthrough.
How long is Dead Space and how many chapters are there?
From my experiences, the Dead Space Remake should take approximately the following amount of time (though differing player skill levels and approaches will obviously lead to some variation).
- Just Story/Critical Path: 11-13 Hours
- Average playthrough/some side quests: 15-18 hours
- Completionist/100%: 18-20 hours
Speaking personally, Dead Space took me about 20 hours to complete its 12 chapters, but I did 100% all the side missions and a lot of backtracking in the process.
Many of the side quests in Dead Space are locked behind story progress though, so you can frequently follow them into dead ends which you can't do anything with, at least until you go back to the story where a new door or pathway can open up after you've completed a main objective. What I also discovered eventually as I played was that most side missions are largely along the route you'd follow as you complete the story anyway, so I might have actually been making things harder for myself by trying to do side missions when they first appear. You simply can't do them until the game allows it, but it's not always clear when that is - there are 'access denied' markers, but I found it a little confusing in a few places. Plus, if you don't check in regularly to see if a mission is now available you can be further away from it.
A core playthrough of Dead Space focusing on just the main story missions will last about 11-13 hours if you really focus on core objectives at the expense of everything else, maybe longer if you're finding it more challenging. The main benefit to doing the side missions are some extra story beats and trophies/achievements. However, the Dead Space Master Override and crew rigs quest will get you a key to certain doors and crates that hold valuable resources and upgrades late game, so that is worth doing.
Overall, Dead Space will likely take you 15-18 hours to finish on a casual playthrough. Although expect that to edge closer to 20 if you aim to see everything and finish every mission.