Here you go:
Frequently: Starter isn't sub'd out until he can't start a play at higher than 90 energy. Once all players in the order can no longer start a play at higher than 90 then the player with the highest energy in the DC is in the game.
Sometimes: Same as above, but 70 energy.
Infrequently: Same as above, but 50 energy.
Keep in mind, it's not finish a play above that energy threshold. Its starting the next play, you can recover energy inbetween plays.