what is the longest single player linear game you have played

Witcher 3 maybe.. Also Nioh took me a while but i checked to be sure Witcher 3 is longer.

Edit: Oh i just saw first comment. I don't know i don't play much of that kind of game i guess. Nioh might be still eligable but if it still doesn't count, the closest thing might be Shadow of Tomb Raider.

Uuuuummmm, honestly, your reply to the first commend didn't make it any less unclear to me as to what counts as linear. ^_^'
I'll just list my most played definitely-not-open-world games in order. Just... pick one of these, I guess?

Darkest Dungeon - 367 hours (I definitely would not call this one "linear", plus I've replayed it several times from the start.)
GOD EATER 2 Rage Burst - 290 hours (This one is mission-based and there are a lot of "optional" missions which don't advance the plot. Even following the main plot missions it's still quite long.)
XCOM: Enemy Unknown - 246 hours (Same as Darkest Dungeon. Too many procedure generated missions for me to consider it linear.)
Recettear: An Item Shop's Tale - 204 hours
All the main Hyperdimension Neptunia games. Megadimension Neptunia VII was the longest at 159 hours. (They are very plot heavy, but the gameplay is primarily dungeon crawling, so... I dunno.)
Giana Sisters: Twisted Dreams - 115 hours (This is a straightforward platformer so it's definitely linear, but there isn't much of a "story-line" going on. Also my playtime is bloated since I liked it too much and kept trying to get the borderline impossible achievement. XD )
Serious Sam 3: BFE - 110 hours (WINNER!... in case none of the above count.)

If we are including a single playthrough, then Skyrim. If we are including multiple playthroughs, Chrono Trigger hands down. Note that I have owned this on Playstation & SNES. So my Steam Hours are paultry compared to the dozens of playthroughs I did of it.

Zelda: Skward Sword
It took me 60 hours to finish it. Absolutely loved it, and was sad when it was over.

AC Odyssey about 170h (includes all DLCs)

Zoombinis a kids game that got harder and harder as you went to the point where some games that are almost 100% luck and chance would murder your entire party and send them back to a checkpoint.to win that game you need to get like 1300+ zoombinis from point a to point b in party's of 16

DOS2 maybe, though it's not really strictly linear.

144 hrs on record for Deus Ex: Human Revolution (not director's cut), if that's linear enough.

For me it was definitely: Indiana Jones® and the Infernal Machine™
I can't say how many hours i spent with it, but it was a very, very long time!
Back then that was 2 CDs!

Gothic Arcania IV - ughh felt like ages to finish that line _____ (this is no serious comment).

Thief (2) - Zombies always stopped me from advancing further. Took me years to beat the series.

Life is Strange ;)

But beside from PC:
Pokemon or Mario games probably have my most linear playtime without repeats.

Final Fantasy 7.

1500+ hours, many, many playthroughs including Item Only, Level 1 challenge, etc.

What game FEELS the longest: Resident Evil 4. Not sure if it really is the longest single player game i played. I must have finished that game 20 times and have all achievements on the Steam version. Resident Evil 6 is also very long (but multiple campaigns so it may not count). Now that i think of it, Final Fantasy VIII is very long as well. All FF games are.