If you just want to maintain your current stamina level, once a year pretty much does it. If your players have bad stamina, or you simply want to raise it (e.g if you play a fast paced tactic), it makes sense to train it twice. I would recommend the all-star break and the off-season, because there you don't have 3 games a week, and thus usually not enough minutes to train properly (Stamina and other team trainings don't rely on minutes played).