A game is being played where eight players can last for seventy minutes. Six substitutes alternate with each player in this game. Thus, all players are on the pitch for the same amount of time including the substitutes.

For how long is each player on the pitch?

40 minutes

Total players = 8 + 6 = 14
Now, the question has told us that only eight players are present on the field at one time.
70 * 8 = 560 min in total.
Now dividing that with 14, we can conclude that each player was there on the pitch for 40 minutes (560/14)