How often does cheating happen in the NFL?

I would say often, in the sense that teams try to find a competitive advantage. And it's not just the NFL, but all sports.

Often. Denver has had 11 players test positive for PED's since 2003, circumvented the salary cap (twice). The Steelers were found guilty of having illegal off-season practices during the 1970's, the Giants used to have someone else take Lawrence Taylor's drug screens. The Colts were caught on tape pumping noise into their stadium in 2007. Jerry Rice using stickum and the 49ers tampering with headsets. On top of that, every team has been accused of tampering with free agents. Like someone else said, every team tries to find a way to get a competitive edge.

Every single game. That's why the refs carry around the yellow flags. Because someone will attempt to gain an advantage on the field.

Offsides, false start, pushing off, pass interference, etc, are all attempts from the players to cheat.

It varies from team to team. Most teams will cheat when absolutely necessary. The Patriots cheat constantly, because they like it and they're not good enough to win without cheating. Tom Brady is the Alex Rodriguez of the NFL.

365 days a year in New England. Cheating never sleeps.

Daily

http://yourteamcheats.com/cheaters/

If you read this you will see how often it happens. Obviously we can all have our own opinions as to the various levels of "cheating" but if rules are broken they should be dealt with and recognized.

Am surprised by these answers I thought it wouldn't happen as often as it does in soccer

Every game

Patriots cheat a lot