I was trying to find out when the swarming on a Pokemon outbreak change to another route, but I could only find this information about "soft-reset". But nothing about the time to reset while playing.
"The gates in Unova have a monitor inside that will alert the player where an outbreak is taking place. During winter, the monitor may occasionally not report on outbreaks, which is due to parts of Route 8, where Croagunk can be found, becoming frozen during Winter and not allowing the Pokémon to be caught there. All have the level range of 15-55.
Unlike previous games, the Pokémon outbreaks can be manipulated by soft resetting the game: turning the console off without saving the game beforehand. When the game is restarted, there will be a different Pokémon swarming."
In Black-White (gen V) works like Gen II ?
"Pokémon outbreaks originated in Generation II. In that generation, if the player registered a certain Trainer in the Pokégear, that Trainer would occassionally call to tell the player that he saw or heard about a large number of Pokémon in a certain area; for the next 30 minutes after such a call (in real time, not play time), the swarming Pokémon would appear in the given area with much greater frequency than the usual 1%."
Best Answer
From the Discussion page of Pokemon Outbreaks (in regards to how to reset the swarm):
Pokemon outbreaks are set by the date, lasting that whole date only after the swarm for that day has been set and saved to the cart.