Scope clauses (part of the contract between major airline pilots and their companies) usually limit the number of regional jets than can be flown by express carriers. I don't know of any scope clauses that specifically limit "block hours" off the top of my head, but obviously limiting regional fleet size will also limit regional block hours. A plane can only fly so many hours in a day.
Once fleet size is settled, some mainlne bean counters, probably in the revenue management departent, determine how many regional block hours are needed. They probably modify the scheudled block hours on a monthly basis. Ever read that fine print on an airline ticket about how flight times are subject to change? Well, that's because the number crunchers will add, remove, or shift capacity based on anticipated demand for their routes between the time a passenger purchases a ticket and shows up for the flight. There are a lot of economic factors under consideration when determining how many block hours to award to a regional airline, and the numbers are adjusted month to month.
If they need more block hours than the current regional fleet can provide, then it's time to acquire more airframes, if the scope clause will allow it. In lieu of that they could add mainlne capacity to regional routes (which is the whole point of the scope clause). If they need fewer regional block hours, it's time to send some RJ's the desert. YAY!