You should realize that the games you consider to have "no hours of gameplay" because they (hours of gameplay per se) can be repeated "ad infinitum" also applies to "other" games "with hours of gamplay". I do understand your questioning, but your introduction of the subject makes no sense, no offense. To put it in other words, let's take a "single-player game" like... say... Fallout 3, for example (correct me if you don't believe that one specifically to be a "GOOD" one, as you pointed out being a prerequisite for a game to even have gameplay hours to start with), so... you'd think that not a single gamer out there would be able to play it ad infinitum, despite being "single-player"?
I'm asking this because, as I mentioned above, you consider a game like Diablo "and the like" to have "no hours of gameplay" when in fact they do, like all other games out there, what do you actually mean by "no hours of game-play" when in fact the very implication that they can be played ad infinitum means that their gameplay hours are being repeated, which in turn means that something is being repeated to begin with, namely gameplay hours, however abundant they are (or aren't). A game like Diablo has a multi-player mode, and a single-player mode, I for one do easily recall playing Diablo II (vanilla, no expansion) in single-player for a good year, if not more, before I even considered moving to its multi-player mode, in which for all intent and purposes the exact same content, story and quests take place, with the difference being that it's on-line and can be played with or against other players.
Alright... anyway I guess that would be subject for another "debate". To answer your actual question, excluding the nonsensical ideology of yours concerning games having, and not having hours of game-play due to their off-line or on-line nature (or due to any other aspects) the only thing I can say is that there are no answers about that for everyone. It is subjective to the extremes. You should realize that some people are content with and feel that games along the lines of Peggle Extreme are complete and accomplished games if they manage to get a single hour of game-play out of them. There are also gamers who buy games like Bad Company 2 and never touch their single-player campaign and stay content with playing their multi-player mode for either a week or a year. And there's players like you who can spend (or even invest) as much as the equivalent of a year of game-play scattered within multiple years in a single game, even if the said game has a multi-player mode (such a Diablo).
If I "answer you" and say that for me, subjectively, the "sweet spot" isn't counted in actual hours but rather counted in "story completion", how would you perceive that? Because, for me, a "GOOD" single-player game (which, for me, means that is story-driven, à-la Knights of the Old Republic or Baldur's Gate or Mass Effect, for instance) has just as many hours needed for its story to be "properly" told and completed without too much cliffhangers or simply unanswered questions regarding the main story arc(s). So I'll take another example which I've played myself recently (for the sixth time so far), namely Dragon Age: Origins (including all but one DLCs, and the expansion pack), usually it takes me anywhere between 45 to 60 hours of game-play or so to "complete" it (I do mean completing every single quests, the DLCs, and the expansion pack), and I feel that it isn't "too much", nor "not enough" simply because by the end (after I'm done with the expansion pack) everything that had to be told in the story and was necessary to be told for it to be understandable was enough and properly done.
But there's another category, those games in which there's basically no story, which means at least for ME that in games like that whatever amount of time I put in them is going to be felt for each minute, simply because if there's no story and/or no characters to hold on to, to keep my attention and my eyes focused on the screen without looking at the time on my clock has to be "fun", but I never end up playing such games for long, even though they do have "hours of game-play", they just happen to have as much as I care to spend time in. A game like... say... Serious Sam or Painkiller, or Just Cause 2 or War for Cybertron... those games do have a story, not that they are complex, or "good", or even "interesting", but I will play those games for as long as I'll have fun playing them excluding the "fun" I'd get out of their story had it been the case. I would be content with playing Painkiller for just one hour before leaving it until my next play session in two months, I will eventually complete it and in the end the accumulated amount of time spent in it might be anywhere between 6 or 10 hours or so, which would be a "sweet spot" for that specific game.
So, yeah, long story short, there's different tastes all around, different people, it depends on all that and the games themselves, and what the developers tried to do with the said games. A developer making a game like Painkiller can't exactly expect even the most loyal fans to play them for 100 hours, even if it would be possible. But a developer making a game like Mass Effect or The Witcher 2 can easily at least "expect" players to play for as long as it is necessary to at least complete the game once, even if there are gamers out there who couldn't stand such games and similar others just by looking at them and wouldn't approach them with a kilometer-long stick after playing them for just ten minutes, which for them was more than enough to conclude that the said game(s) wasn't worthy.
Everyone who cares answering your question will end up with their own self-satisfying answers to which you will inevitably disagree at some point or another anyway. I mean don't get me wrong, I like discussing different opinions, this is a forum and we come here for a reason, but this is way too subjective.
Facebook
Twitter