I think it depends on the nature of the game, and the mindframe of the gamer at a specific time.
Games light on plot, and gameplay intesity are good in short doses.
Games heavy on exploration, (in terms of size, space, or things to do (e.g. Fallout...)), plot (I.e. JRPGs, Bioware epics, Pointyclickers (for me anyway as I suck at them (but I do love them!
))), or gameplay intensity, (RTS, Might & Magic, ect.) need several hours, (or even days) of investment to get that certain level of satisfaction.
I know I have a horrible habbit of hoarding games, (I hardly let a good bargian slip through my fingers
)
Current No. of games unplayed (approximatley):
50 xbox games
20 xbox 360 (+ About 40 indie games, and about 20 arcade titles)
15 PS2 games (Odin Sphere, SMT 3: Nocturne, Summoner 1+2, Dark cloud 2 and DMC being recent aquisitions)
100 pc games
But my problem is I crave genres of games at different times, as well as lead a unpredictable lifestyle, (sometimes I have only a few hours here and there, sometimes I have days, weeks, or even months of free time, (I got Uni you see!), but also I live with my family, so different gaming areas are available at different times, (so it hard to satiate a particular craving)).
My second hobby also leaks into that time as well, (self-teaching myself at guitar at the moment, (slow progress, (especially so since I'm trying to learn on a right handed guitar, (I'm naturally left handed, (though I suspect that may be slightly selective as I can only use a pc mouse and scissors with my right hand... :/ )))).
(As well as my third, (talkin' about games on forums), and what was my fourth (trying to learn to make videogames (I did have XNA on my lappy before it got wiped, (unsuprisingly never got round to learning anything from it), as well as Game Maker 8, and Unity, (for the record, I also tried to use AGS and RPG maker when I was younger as well (didn't suit me all too well :/)
I'm just glad I'm not a reviewer, else I just wouldn't get anything done at all! XD
To me, getting 10% of players to complete a game well over 20 hours long is a darn good achivement.
Not every game can draw people in for long periods of time, (us gamers are exposed to just as much external stimuli as average joe! XD).
(Not every game can be Minecraft! XD (JOKING! (or am I....?)))
EDIT: To clarify. I don't buy games as they come out, (£40 just seems excessive to me now that I buy online). I tend to buld up a mental wishlist, and hunt them down over time.
(also I am not rich. I work as a till monkey at Tesco. I just don't buy clothes or alcohol, and take advantage of Ebay, Shopto, Steam and GOG (also I usually forget about most games until, like 6 months later...).
Analogy: Sometimes we want candy, sometimes we want a roast, and occasionally its nice to have a 3 course meal.
But not many people want to just eat candy, or roasts, or a feast all the time.