Josefer,
You've posed an interesting question, which unfortunately I don't have a definitive answer for. The amount of time your department chairs need to put into their advisory boards depends on several factors; a) their job description, b) expectations of senior management, c) training on creating and managing advisory boards, d) perceived and actual value received from their advisory boards; and e) their willingness to be more than average and just go beyond just complying with accreditation requirements. If you can answer these, then you may come closer to the amount of time that could/should be spent on creating and managing successful advisory boards.
In my past, I co-chaired a national advisory board with the chairman of my company. Initially, over a half-year (we met twice annually) I spent a couple of days planning and organizing agendas and collecting materials for meetings to assure they were productive. As I became more experienced, it took less and less time to prepare and to retain our effectiveness; the same would apply to your department chairs. Initially, if you could allocate ½ day with them to help them plan organize and agree on common goals, which should be sufficient. Taking the information you get from this course and applying it to your situation in a “training†capacity should help you achieve the efficiency you're looking for. As with any worthwhile endeavor, the initial efforts require additional time, but perhaps they just don't know how to get the most out of their expenditure of time and energy.
Assuming you have two advisory board meetings per program, per year; each chair could devote one hour per week towards making their board more effective. Taken in small bits, with a clear objective and accountability, they should be able to enhance the effectiveness of their boards without too much difficulty.
R. Roehrich