Even though Volcanic Rush affects only attacking creatures, the bonus will remain for the rest of the turn.

Only creatures that are attacking as Volcanic Rush resolves will receive the bonus. In other words, casting it before you declare attackers usually won’t do anything.

An “attacking creature” is one that has been declared as an attacker that combat, or one that was put onto the battlefield attacking this combat. Unless that creature leaves combat, it continues to be an attacking creature through the end of combat step, even if the player it was attacking has left the game, or the planeswalker it was attacking has left the battlefield.