To set a mass break (e.g. lunch break), go to Conferences > Modify Schedules > Advanced (on the right-hand side of the screen).
Sort by the conference name and date, choose the time interval, then be sure to hit Submit.
Please note: the break you set has to correspond to the time interval - that is, it needs to be equally divided by the time interval.
For instance, if the time interval is 20 minutes, your break can only last 20, 40, 60, 80 etc. minutes.