Average time per move is calculated and adjusted each time you make a move in an online chess (correspondence) game. When a move is made, the total number of seconds passed since your opponent moved is added to your "total move time" and your "total move count" is incremented by one. (Seconds passed while on vacation are ignored.)

Note that conditional moves count towards the average time per move. A conditional move counts as 0 seconds and therefore will lower your average time per move.
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Your average time per move is simply "total move time (seconds)" / "total move count".

Lets look at a few examples... Assume Joe has a "total move time" of 0 and a "total move count" of 0.
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Joe makes a move in one of his games 18 hours into his clock. Joe makes another move in another game 1 hour into his clock. Joe makes another move in another game 48 hours into his clock.
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"total move time" = (18 + 1 + 48) * 3600 = 241200 (seconds)
"total move count" = 3
"average time per move" = 241200 / 3 = 80400 = 22.3 hours

Now Joe (from example one) sees that two of his opponents have already moved so he quickly goes and makes a move in each game. In one game only 3 minutes has passed and in the other game only 1 minute has passed.

"total move time" = 241200 + (4 * 60) = 241440 (seconds)
"total move count" = 5
"average time per move" = 241200 / 5 = 48240 = 13.4 hours

Now Joe realizes that he's running out of time on a couple really old 14 days/move games, so makes his moves in both of those games. In one game 13 days has passed and in the other game 12 days has passed.
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"total move time" = 241440 + (25 * 86400) = 2401440 (seconds)
"total move count" = 7
"average time per move" = 2401440 / 7 = 343062 = 95.3 hours