Press Reports (such as this one) have stated that the Obama campaign has purchased half-hour blocks of time on at least NBC and CBS to broadcast a political infomercial to be aired at 8 PM Eastern time on October 29. Some reports indicate that other broadcast and cable networks will also be broadcasting the same program. Did the networks have to sell him the time? In fact, they probably did. Under FCC rules, Federal political candidates have a right of reasonable access to "all classes" of time sold by the station in all dayparts. This includes a right to program length time, a right that was affirmed by the US Court of Appeals when the networks did not want to sell Jimmy Carter a program length commercial to announce the launch of his reelection bid. Because of this right, the networks often had to sell Lyndon LaRouche half hour blocks of time to promote his perennial candidacy for President.
How often do networks (or stations) have to make such time available? They only have the right to be "reasonable." While what is reasonable has not been defined, the amount of time that will be requested will probably be limited by the cost of such time. Even were it not limited by cost, the FCC would probably not require that a broadcaster sell such a prime time block more than once or twice during the course of an election – and given the late stage that we are in the current election, it seems unlikely that more than one such request would have to be honored during these last few weeks of the campaign. Stations do not need to give candidates the exact time that they requested – so the rumored reluctance of Fox to sell this precise time to the Obama campaign because it might conflict with the World Series would probably be reasonable – if they offered him the opportunity to buy a half hour block at some other comparable time.