I use the term "popping rate" to refer to the amount of novel/interesting/useful material gained in a given time period. If you're used to microwaveable popcorn, you know that as the popping rate decreases past a certain point and pops become rarer - if you don't take the popcorn out of the microwave, it will fry. I think the same goes for my brain when it is trying to suck up knowledge. If I'm watching a really interesting documentary, reading a good nonfiction book, or listening to a captivating talk, I can almost feel the new knowledge and facts pop and fill my brain. I consider the time well-spent if the popping rate is above a certain threshold... however, if the pops become too rare, I feel my brain frying under lack of stimulation. That is when I know to turn off the TV, put down the book, or zone out of the talk... and pursue an activity with a higher popping rate.
In fact, I've found that quantifying the informational/factual content of something using some notion of pops per minute or pops per hour (referring to # of novel bits of information/facts gained during that time), provides a useful frame of comparison between activities.