Up until 6 months ago, there were two intermediaries between me and the QSL bureau. Now there is only one. But figure one to three months between the time when the cards get to the QSL bureau and the time it gets to the sorter. Depending on his schedule, this may occur anywhere from 1 week to 1 month after he gets the cards from the bureau. If you add all those times up, you'll see why it takes an average of 6 months to a year.
Different sorters work differently.
I used to send ALL QSL cards out if the ham had an envelope on hand. But after getting a number of nastygrams complaining about only one QSL card in the envelope, I have changed my methods. OR if I've held the one or two cards for 90 days. I figure if I've held it for 90 days and there are no more coming in, the ham probably still wants the cards. I have a few hams who have specifically requested that I send their cards every month regardless of how many they have, and I do this.