The best game I can make right now probably won't hold up for 60 hours of gameplay,
Nothing says a game has to have 60 hours of content. Sure, $1 per hour of content and a $60 price point for a new AAA game are pretty standard, thus the expectation of 60 hours+ gameplay. But that's not written in stone anywhere. And fewer and fewer titles these days deliver on that one hour of gameplay per dollar spent. Either they are small, and deliver less than that, or they are huge, and deliver hundreds or thousands of hours for $60.
and I can think of some games that have great gameplay, imho, that didn't hold up for 20 hours. Not because it was lackluster, but just simply time investment requires a certain amount of depth to plumb.
I don't follow... they were too shallow? IE great gameplay, but not enough of it? At least not enough to want to do it for more than 20 hours?
I understand. What I was attempting to refer to with my earlier comment is that you should be able to look at the gameplay you've created and have a strong idea whether it's a best fit for the hundreds of thousands of hours or the less than 60. A great example, for me, is the Arkham game series. I've enjoyed each game, but as the 100% completion has gotten bigger and bigger, finishing the games has gotten more and more tedious and less fun and engaging. The depth of the gameplay doesn't last long enough. And while mileage naturally varies on such things, I think there's a ballpark or sweet spot that can be aimed for.