I've speculated that up to half a game is commercials. Anyone who has the DirecTV package of all the games each Sunday can attest to that, as on average half the games at any given time are on commercial break, often showing the exact same commercials only a few seconds apart.
Here's the basic formula that I've discerned: The first series of a game is shown; if it's more than a three and out they cut to break after it's over. Then, after every score there's a commercial break, then another one after the ensuing kickoff. Every time a team calls timeout, there's a commercial break. Change of possession? Comercial break. Every time a player writhes on the turf and won't leave the field of play (or acts dead, almost always blacks), there's a commercial break. A replay challenge? Commercial break. Not enough replay challenges or pseudo-dead players on the field? The referee then will call timeouts so that the network can go to commercial break.
If the allotment of commercials is shown, then games don't cut to break at the usual timestowards the end of the first half and second half.
Some years back the rule stopping the clock when a player goes out of bounds was changed (except for at the end of a half), not to speed up the game but to allow for more endless commercials. I've tried to search a couple of times to find theamount of timethe averagethree-hour game is on commercial break, but whatever the exact percentage is, it's obscenely high.