a distinguished presentation of 45-60 min, after this some time for discussion, break. Then two lectures of 30-40 min. one after the other, then a shorter break, and then in the same manner. Mostly is the problem that the overall time of conferences is too long for some people.
I agree with Hein. Some distinguished speakers keep on giving lectures completely ignoring the time allotted to them which eventually disrupts the smooth flow of the schedule.