IMO, it should generally be left up to the PRO as they see fit on any given day. The PRO should make the decision based on the level of racing (national vs. fleet level, etc.), the expected weather conditions during the series, the number of days of the series, the desired total number of races, and the overall endurance level of the racers.
It is not a simple decision and there is no general guideline that will work for all regattas. If your PRO is "in touch" with the racers, they should have a good understanding of what is desired by the sailors. Placing too many restrictions on the PRO can limit the overall quality of the regatta.
That being said, any limitations should be clearly stated in the sailing instructions and even in the NOR if possible.