One Tenebris Somnia playthrough takes roughly 4 to 8 hours, depending on how fast you solve the puzzles. Seeing all four endings takes at least three playthroughs, so plan on well over ten hours for the endings alone.
Tenebris Somnia how long to beat: one playthrough
The descent is a single unbroken building with no chapter select, so backtracking to a floor for a puzzle you skipped adds to the time rather than costing you a menu trip. Puzzle solving is where the spread comes from: the film school on Floor 0 alone holds three puzzles, and the Floor 1 fly tiles, the bar bottle pitch and the elevator colour order all change every run, so they cost a little more each time. The route those hours are spent on is in the walkthrough.
Seeing all four endings
The two main endings — SOLVE ET COAGULA and MAGNUM OPUS — fit on a single save file. Make a manual save right after forging the Aurifer, finish that file for one ending, reload it and take the other branch.
The two hidden endings, VIVUS EX MORTIBUS and RENUNTIATIO, each need a fresh new game. Counting the reload as one playthrough, that is three playthroughs and a bit over ten hours in total. What each ending needs is in the endings guide.
Going for 100%
Add more on top of that. The missable achievements need their own runs: Wrench Queen requires a full playthrough with the wrench only, Exhaust Pipe needs the Fumus fight with the iron pipe alone, and He Just Wanted a Hug needs a firearm. A practical plan needs a wrench-only save, a pipe-only save and a firearm save on top of the three runs the endings use. The full list of the 24 achievements is in the achievements guide.