Around 4 years or so until 2018. Since then, about a week or two.
However, I rarely finish games these days. For the most part I play the same 3 games online, and every blue moon I'll play something else. Back in the day, I used to beat every game I owned. My list of games I never beat includes a lot of what was considered the best of the last two gens.
RDR2
GTA5
MGS5
Dishonored
God of War
Bioshock
TLOU2
XCOM
Persona 5
DOOM
etc.
This gen I never beat Psychonauts 2, Ratchet & Clank PS5, Deathloop, Cyberpunk, Returnal, etc.
The only games that I consistently beat are indie shyt like Guacamelee and Limbo lol. For whatever reason I don't have the patience for story-driven videogames anymore and especially if the gameplay loop gets repetitive within a couple of hours.