Unseating an incumbent member of Congress is a difficult undertaking at best. Given the party demographics of McCarthy's district (Long Island, NY) and her party affiliation (D), I'd say, realistically, the seat is hers until she decides to retire.
I live on long island just 2 minutes from her district (unfortunately I can't vote her out). Don't confuse Nassau county Long Island politics with those of New York city. Republicans have until very recently had an almost unbreakable hold on Nassau county in fact before McCarthy the reps were:
Henry J. Latham, Republican, January 3, 1953 - December 31, 1958
Seymour Halpern, Republican, January 3, 1959 - January 3, 1963
John W. Wydler, Republican, January 3, 1963 - January 3, 1973
Norman F. Lent, Republican, January 3, 1973 - January 3, 1993
David A. Levy, Republican, January 3, 1993 - January 3, 1995
Daniel Frisa, Republican, January 3, 1995 - January 3, 1997
For nearly Forty years this district was uninterupted Republican territiory. Infact my rep is a republican though not really pro-gun. Some of her races against republicans have been very close.
But with the way the country has been moving away from the republican party in the last election I don't see her being unseated in 2008. But that's more a function of the back lash against Bush than her having an unbreakable hold on the political scene here.
Ironically she lives near my favorite gun store.