I usually play every game to completion, if it's any good at all and sometimes even if it isn't. 

I also tend to have this (arguably bad) habit which, in the not-as-rare-as-you'd-think-but-still-kinda-rare occasions I start a game I don't like, will end up with me avoiding playing anything at all until I finish that, sometimes for weeks on end

That being said I did drop the first Sam and Max game and postponed Gabriel Knight 1 seemingly indefinitely

I don't really have any adventure games I didn't finish. I did do a primary run of "Don't Escape: 4 Days to Survive" once but never went back to continue...

Oh, wait, I'll never finish "Blue Prince" probably... 110 hours and I'm on day 65, reached a point where it has become too difficult, even with a walkthrough...

Lots of other genre games I never finished, though. Like AC 3, Red Dead Redemption, FF 13, GTA 4. Not due to lack of interest, but because newer games squeezed through and I just got a lot more into regular adventure games again.