Believe it or not I make an effort to finish any game I start and only that game until I see the end credits. I am probably in the minority as, judging from the weekly "what games are you playing" post, I always see folks list a multitude of games they're playing simultaneously. The only exception to this are shmup's as I need to "mix things up" whenever I find myself stuck at a particular boss or area.
If I stop playing the game it's usually because I am either bored or not having fun. That is the point when I'll usually sell or swap the game and it remains unfinished.
I'm very stop and startey with games even if I love a longer game I tend to get busy with stuff I have to do then can't get a chance to get back to finishing it until a few months later. Have noticed this gen I am much quicker to just give up on games if I'm not enjoying them or not try to finish them. Just gave up on the new God Of War a few hours in when I couldn't stand playing it any longer same for Horizon Zero Dawn.
Those long 100 hour games always catch me up always tend to be 25% at the very least slower to complete them then everybody else says it takes for them to finish the game.
Give it 2~3 hours in, if by then I'm not feeling it, I'll probably not gonna finish it unless I have hope it gets better.
But then again you're talking to someone who beat Family Guy something something Multiverse because he was bored, don't take my word for it. Sometimes you just want to see that trainwreck in person. And I've been pretty vocal about how much I dislike a LOT of things about Xenoblade 2, still somehow was able to finish it AND get all the non new game + gacha BLADEs, even though I was pretty much feeling off about the whole thing more often than not, it was Xenoblade, I paid full price, I had to see it through.
So, money spent is actually a very good motivator, somehow I've beaten every 3DS physical retail game I've bought besides Mario Kart 7, and that's counting Paper Mario Sticker Star! AND THAT GAME... Is certainly something I've been warned about. But hey sometimes you got to press on if you ever intend to finish a game, you'll be at least able to say "Yes, I can talk about that one, I've played it to the end, dammit!", and it was like 50% off, surely it couldn't be thaaat bad, right?... Hahaha, ha. (If you have a 3DS, the best Mario RPGs are probably the remakes for Bowser's Inside Story/Superstar Saga, just, just go get those instead, please)
I don't finish a lot of games these days. I think it's because I enjoy returning to my favorite games over getting into new ones. Like, yeah I could play Persona 5 which is entertaining and engaging and I do want to see where it goes. But what do I do? Boot up an old fave instead because I usually want to let my brain rest and that's easier to do with something I know the ins and outs of already. I don't need to pay attention to the story, or learn new gameplay mechanics. It's just so much more relaxing to run through the first chunk of a game I know and love rather than focus up and learn to love a new game.
On rare occasions a game will come along and really suck me in, such as Blasphemous. Even rarer are the games that compel me to continue playing despite being horribly flawed, like FF7R.
I used to have this problem where I wouldn't want to start new games until I finished the one I already started, but when I stopped wanting to play the game that was keeping me from playing something new it became a problem. So now I hope between games a lot. I let my desires guide my choices when it comes to what I want to play. Maybe I'm into old school FPS games one month, and then I'll get into an MMO for the next, or go on a JRPG kick. Who knows? Certainly not me.
I play them for as long as they entertain me and no longer. I'm not one of those people who feels a lack of accomplishment if I don't finish a game.
That being said I rarely finish games completely. I find very few game are worth completing as most show all their cards by halfway through the game and introduce nothing new from then on. There has to be something compelling to counteract that, like an excellent story (Metal Gear Solid, Chrono Trigger, etc.), elements that create high replayability (Way of the Samurai, visual novels with multiple routes like the Zero Escape series, etc.) or something to that effect.
I get stressed if I think I’m near the end of a game, I’m not because I’m only 40 hours in, and will stop playing and switch to something else. It is nice because all of the unresolved threads get muddled up in my head cannon.
I'm a bit of a trophy/achievement chaser so I'll definitely try to finish a game unless I really dislike it. Last gen i was a single format gamer which made concentrating on one or two games at a time more manageable, owning an Xbox and PS4 this time around means I'm spreading myself thinner.
Games I startup with the intent of actually playing, pretty high. Sometimes though I just boot up a game to see what it's all about, i.e., the other games in that bundle you bought. Much lower rate on those.