How much broadcast time did it take for Planet Namek to blow up?
Interestingly enough, the TVTropes link I linked in the comments had the answer


  Taken to insanity during Goku's fight with Freeza. Freeza destroys the
  core and gives the planet five minutes until implosion. Ten episodes
  (approximately three hours of screentime for each of the scenes that
  are playing out simultaneously) and over three hundred lines of dialog
  for the two fighters later the planet finally collapses.


So there you go.  


10 episodes
3 hours (roughly)
100 lines of dialog


although, apparently it was


  Lampshaded snarkily in the dub. With ten episodes to go, Frieza has a
  line to the effect that the planet is "a tough one... it'll probably
  last another two minutes."