The regulation clock is 48 minutes.
NBA regulation consists of four 12-minute quarters. Each overtime lasts five minutes; halftime lasts 15 minutes. Further overtime periods are played if the score remains tied.
Quarter breaks are part of the viewing time.
Rule 5 allows 2:30 between the first and second quarters, the third and fourth quarters, and before overtime for local games. National TV games use 3:30 for those quarter breaks and 2:30 before overtime.
Read a clock without guessing the finish time.
If the fourth-quarter clock shows 6:00, six minutes of regulation playing time remain. It does not mean the broadcast will finish six minutes later. A tied score can also take the game beyond regulation. There is no single exact wall-clock duration to attach to every NBA game.