EDIT: I opened Mega Database 2012 and found there were 515539 games ranging from the 1400s to 2011. As I couldn't find an efficient way to parse the data, I just used the median move value (I hope it's normally distributed. It's probably not though as the largest number of moves was 277... oh well). The median value was 37 moves, but I'm sure that it's a ...
I did this graph based on 731,000 games played on the free internet chess server by players with rating above 2000 during the last 13 years with non-blitz time controls.
The average length is 79 half moves, the median is 70 and the mode is 51. There are also a lot of very short games due to people resigning early.
Update: As it turns out, the above curve ...
An interesting question with which I've done significant exploration. My program Symbolic can generate random games using the /dev/urandom pseudorandom generator to any length. For a recent run of ten million random game, the average length was 342.064 ply.
 rg 10000000
Checkmate 1527544 0.152754
FiftyMoves 2241451 0.224145
Insufficient 5358614 ...
We can make a small analysis from a theoretical point of view:
Let us call 'n' the number of pieces on the board.
In a match between players of similar strength, it is expected to find ~n/2 pieces for each side.
When it is White's turn to move, they have 64-n/2 available squares, n/2 are occupied by rival pieces.
That is, fc=(n/2)/ (64-n/2) is the fraction ...
The average length of chess games depends highly on the reflection time schedule since decisive errors happen very often under time pressure. Cf. https://ingram-braun.net/public/research/parlour-games/article/statistics-length-chessbase/
Sahovski Informator publishes approximately 700 of the Best or Most Interesting games of the preceding several months, played by Grandmasters around the world. I have found the average number of moves in several volumes to converge at between 43 and 44 moves per game. This is, of course, for Grandmasters in tournaments or matches, so they are not giving up ...
The answer, as hinted in the other comments, is that there isn't a solid definition. Unlike @Mikhail Tal's answer, though, the number I had always head was 20 moves (40ply) or thereabouts.
Those who are familiar with 30 as a boundary might think of this as too short, but as an example the "Opera Game" was only 17 moves and the "Immortal Game" for all its ...
If you are talking about the embarrassment of losing in so few moves that it counts as a miniature then the answer is 20.
If you are trying to scrape together as many short games as you can then the answer is 30, but the one that really counts, that really hurts is the 20 move one.