Depends on the situation. I try to use it shortly after I get it so that it can recharge and I can get a 2nd one before the end of the match. Used to have a problem of saving them waiting for the perfect situation to use it and sometimes that never happened. I try to get at least 2 enemies together. If I see a Titan bubble the Nova Bomb gets used right away even if there is only 1 person in the bubble.