Can someone explain the length of the Olympic soccer games?

There are 2 things to know timekeeping in soccer:

1) The official time is what the referee keeps on his watch. The running clock on the scoreboard is for information only. The half ends only when the ref blows his whistle to end it, not when the clock hits 0:00.

2) The clock never stops for injuries, goals, substitutions, fouls etc., as happens in most other timed games.

That said, the "fourth official" who in on the sideline between the team benches will keep track of playing time lost during the half, mostly due to injuries but possibly for other reasons as well. In the last couple of minutes of the half, he raises a board showing how much "stoppage time" the referee should add at the end of 45 minutes. In most matches, this is between 2-4 minutes but can be longer. The referee then adds this time to be played. The referee as the official timekeeper then blows his whistle when he is satisfied that the additional time has been played.

It is the same regulations as all soccer/football games whether it is Olympic, World Cup, MSL, Youth, etc... There is a set amount of time for each half, it is none stop so extra time is added at the end of each half for all the stoppages of time whether it is substitution, injury, etc.. usually it is 2 to 4 minutes unless there is a major problem on the field.

They add time back for moments during the game when play stopped. In American football the clock is simply stopped. In worldwide football/soccer, the time is added at the end.

The clock doesn't stop during the match so the ref adds time on for extended stoppages during the half (e.g. substitutions, injuries, goal celebrations, etc).

It's purely up to the ref how much they add on (though sometimes the point of controversy from managers).