What, if any, is the accepted "norm" for the required length of continuous sobriety in order to chair AA meetings; any difference for chairing "open" meetings versus "closed meetings, " "Big Book Studies" versus discussion meetings, etc. Is there any rule-of-thumb beyond a group conscience decision?
There is no rule as to how much sobriety a person should have to chair meetings, it varies from place to place and is usually determined by the group conscience. One group may suggest at least 3 months while another might suggest 6 months or more.
Each group is autonomous, as stated in the 4th Tradition therefore it should be up to the individual group to decide what's best for them.See this answer on chairing meetings also.