The Flash (2014– )
FAQAdd to FAQ (Coming Soon)
For the first crossover you do not. Each episode tells a largely self contained story, with characters from each show simply traveling to the other one's city to fight a villain.
In the season two crossover that begins with the Flash episode "Legends of Today" you probably should watch the Arrow episode "Legends of Yesterday" as the two episodes tell one story. If you don't watch "Legends of Yesterday" then you won't know how the story from "Legends of Today" ends. These two episodes also lead into the spinoff series "Legends of Tomorrow". Edit (Coming Soon)
There is no exact speed but as every episode premieres, Barry gets faster and faster and eventually can travel through time.
In the episode 'Tricksters' Barry / The Flash is forced to run over 600mph otherwise a bomb attached to his wrist by The Tricksters will explode. Barry removes the bomb via Harrison Wells' request , vibrating through a wall and leaving the bomb on the other side.
In addition, in the episode 'Fast Enough' Barry / The Flash makes the decision to travel back in time by creating a wormhole to the past by running at super speeds. Dr. Martin Stein adds that he will need to be running at Mach 2 to be successful, any slower will result in his death. He then runs this speed and is successful in traveling back in time. Mach 2 is about equal to 680.58 m/s or 1,522.41 mph. Edit (Coming Soon)