What is the longest you have played one single-player game?

[Lassr]

According to Steam and Epic her are my games above 100 hours. Back in the days of DOS and CDs, the original Civilization, Sid Meier's Pirates, Master of Orion, Madden, MVP Baseball 2005 (which I still played until windows 10) is probably in the several hundreds of hours.

XCOM 2
806 hrs on record

Sid Meier's Civilization VI
402 hrs on record

EPIC-Phoenix Point
391 Hrs

Stellaris
346 hrs on record

XCOM: Enemy Unknown
255 hrs on record

Silent Hunter: Wolves of the Pacific
123 hrs on record +100 or so hours before Steam.

Galactic Civilizations III
137 hrs on record

Fallen Enchantress: Legendary Heroes
129 hrs on record

Sid Meier's Civilization V
115 hrs on record

[Sepiche]

I'd be typing for a few hours if I added every game over 100 hours.

These are my highlights though.

Stellaris / 1773.6

Total War: Warhammer 2 / 599.2

Total War: Shogun 2 / 453.5

Europa Universalis IV / 398.4

Total War: Empire / 376.8

Total War: Rome 2 / 306.2

[Fardaza]

Steam says I have over 1,000 hours in Xcom: Enemy Within, XCOM2, and Skyrim.

All the Civilizations would probably be up there as well as Heroes of Might and Magic.