How much extra time do you think NBA players actually put in?
I honestly don't think that every NBA player is out there bustin his azz during the season/off-season to get better.
They are 92% genetically gifted and 8% hard work/dedication to make the league first off. There really is only a handful of people in the world that a) play basketball b) have the natural athleticism to be an all-world athlete.
I think that almost every NBA player gives it 100% during games. May not always look 100% (e.g., James Harden) at all parts of the game...but when you are playing a sport you are either playing or not playing. Not really any willpower needed...just a matter of endurance and mentality.
Practicing on the other hand is purely willpower. Some people/players have it and some don't. It's probably similar to the workplace. Some people just work harder than others. Everyone did what they needed to do to get employed...but some people care much more.
Let's say there are 400 players in the NBA. Out of those NBA players how many do you think dedicate 4 (or more) hours a day to becoming better at basketball. This doesn't include playing the game, or mandatory practices. This does include stretching, watching video, shooting around, hitting the weights, etc.
To be fair let's compare it to a real job: How many players average a 20 hour work week (not including mandatory practice + games)?
My guess is 10%. I'd say 75% probably average between 2 to 3 hours a day (including stretching, weights, watching video, and shooting around). With the last 15% putting in less than 2 hours a day.