How often do you think a Manager should provide staff training on food and beverage knowledge?

As often as possible really. 

"AS OFTEN AS NECESSARY"

Employees,should know what theyre doing,.For example,in restaurant,staff should know everything,THEY SHOULD KNOW HOW TO ANSWER "WHAT,WHERE,WHEN,HOW "..Thats why we have "BRIEFING" before the operation start, to remind not just our staff but as well as ourselves, why were here in this industry.."

Is often as you see necessary. To provide the best customer service, your people should know everything they are talking about. Give them quizzes during training, and couple of days after (without training) to make sure they didn't forget. Make sure all of their questions are answered, as well. Be a teacher, and a leader, not just a boss, because that will help them be more willing to learn, and feel more comfortable asking questions. 

A manager should not drink too much in a bar because managers need to take care of staff.

If he or she has relevant skills and knowledge.