The average length of the chess games of the first 14 world chess champions range from a low of 34 (Spassky) to a high of 42 (Lasker). So, at least one measure of long is established by numbers greater than these, I would suppose. Here on chess.com the average blitz game is about 30 moves (61 total ply). Not surprising that amateurs have a decisive outcome in fewer moves than GMs, though the fast time control may be influencing that as well.
Long Chess Games
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How many moves do you feel a long chess game is? I say 60+