that's the key. does it feel like fun, or work? is it relaxing or entertaining? am i experiencing an interesting story?
i also try not to spend money on games unless i am sure it's going to be good, or the amount is small enough to avoid the sunk cost fallacy. i'd want to get my money's worth.
whenever i get the feeling that i should be doing something more productive then that means that the game is not enjoyable enough any more and it's time to stop.
(note: eve online is probably not fun for most people, lol)