Constantly. Really, there should be some training at every meeting of your organization. You should devote 10-15 minutes in each meeting to training your officers and members about some aspect of student government.
At a minimum, you should have training "retreats" at least three times per year.
But no one gets really good at anything by doing it just three times a year. A musician practices constantly to become excellent. An Olympic swimmer practices daily to become the best in the world. A carpenter practices his/her craft and gets "rusty" by not doing the work regularly.
It's no different for Student Governments. We can't expect to be really good at Student Government unless we are constantly "practicing" and "training."
ASGA's MISSION STATEMENT:
The American Student Government Association will provide all Student Government leaders and advisors nationwide with networking, research, and information resources and will teach them how to become more effective, ethical, and influential leaders on their campuses. ASGA also will promote the advancement of SGs, conduct research as the nation’s only “SG Think Tank,” and advocate the importance of having a vibrant, autonomous Student Government organization at every institution in America.