when do airlines change ticket prices

All airlines change fares continually. Real fare changes may happen 3-4 times a day,depending on the demand for that flight and the amount of time left to book.
But inventory availability can change the prices in a minute or even quicker
In the USA reservations are held until 12 midnight Pacific Time (at the most). What happens to unpurchased reservations? They get put back in the system.

I believe this is how it works. At 12 midnight Pacific Time the system cancels out unpurchased reservations. The computers update and this takes time. By 2 a.m. or 3 a.m. Pacific Time the fares reappear online and for sale again.

From http://askrick.farecompare.com/2008/01/15/when-do-airlines-change-their-ticket-prices/
Domestic U.S and Canada Airfare Distribution

Domestic U.S. and Canadian airfares are distributed to subscribers (reservation systems) on the following schedule and are loaded approximately 2-4 hours after distribution into reservation systems (except on FareCompare.com which sends out free airfare email alerts immediately):

* Weekdays - 3 times daily at 10:00am, 12:30pm and 8:00pm Eastern Time
* Weekends - 1 time daily at 5:00pm Eastern Time (loaded after Midnight the next day into reservation systems)

International Airfare Distribution

International airfares are distributed to subscribers (reservation systems) on the following schedule and are loaded approximately 4-8 hours after distribution (except on FareCompare.com which sends out free airfare email alerts immediately):

* Weekdays - 5 times daily at 1:00am, 5:30am, 11:00am, 3:00pm, 8:00pm Eastern Time
* Saturday - 2 times at 5:00am and 5:00pm Eastern Time
* Sunday - 2 times at 5:00pm and 8:30pm Eastern Time