Gopher,
Under the "stale complaint rule" the FAA has 6 months to begin an enforcement proceeding against you.
Here's the kicker though; even if they miss the deadline, they may go right ahead and proceed against you. They do this. Unbeleviabley, it has been upheld by in an appeal to the full board of the NTSB (split decison) At least one person has appealed beyond the NTSB to the DC district court of appeals. Only there, in a real court before a real judge was the FAA required to abide by their own rules. Truly unbelievable. So yes the deadline is 6 months, but the FAA may try to screw you even when they *know* they are wrong, and to go through the NTSB hearing, appeal to the Full board, then appeal that ruling to a real court is a long and expensive process.
Also to second what the others are saying, if you got something going on, and this is more than idle cuiosity, get an attorney.