20880302_ferences__July_2014_FIDE_Laws___FIDE_rule
[0] 6. [USCF rule] If an illegal move is not corrected within ten moves (or within two moves in a sudden death time control in a time pressure situation), the illegal move stands. [FIDE rule] 
There is no limit how far back in the game an illegal move may be corrected.
The United States Chess Federation - USCF - FIDE Rule Differences (July 2014 FIDE Laws)
FIDE rule