the prices on SOME items may not change for a variety of reasons... IPO prices are fixed through the day they are offered, until reset that night. Movie stocks that open that week are frozen on Fridays of opening day, and are frozen until the stock adjusts on Sunday afternoons - and the price does not move until reset that night, even though you can buy or sell those opening movie stocks after they are unfrozen on Sunday.
There's also a chance with some "blockbuster warrants" that as they grow closer to delisting, that the price may not move much (that you notice) the reason being, that everyone who was going to buy the warrant, probably has, so the price may not move much as it draws near to delisting...
Also, the patented software HSX uses to calculate stock 'movement' is set so that no one really knows how many shares of a certain stock it takes to buy, sell, short, etc. to make the price move up or down. I've seen the price on a stock that's priced at $200 stay the same when I sell it, and then at other times, when I do the same thing...the price drops right after I sell it...
There is 'no' set time for when stock prices change...the exchange is open 24/7, so the simple answer is - they can move any time (with some of the above exceptions) and any hour...