In a National Basketball Association statistician manual, it says that an assist should be “credited to a player tossing the last pass leading directly to a field goal, only if the player scoring the goal responds by demonstrating immediate reaction to the basket.” This sounds simple enough, but when you try to apply this definition during game play, the definition becomes very vague as there are no details on the extent a shooter should take after receiving the pass. There is no indication how much dribble, steps, pivots, moves, or fakes can be taken between the pass and shot before it can be credited as an assist.
taken from:
http://basketball91.com/coaching/basketball-assist/
i think on 2k though there is like a timer on it. i think on 2k it depends on how fast they take the shot, if its within a certain time they count it as an assist.