How often do stock prices change in GTA V?

The prices with the LCN are affected by what you do in the game. The prices on BAWSAQ are influenced by what other players are doing within the game.

Back to the LCN stock market though. There will be missions in the game that will cause the stock prices of companies to change, so if you know which company to invest in before the stock prices increase/decrease then you can make a lot of cash in short period of time.

If you are interested in more information I've found an excellent guide on how to make money from the stock market.

The LCN stock prices update every thirty seconds, unless the game has been on for a significant period of time. As of this post the exchange will sometimes freeze. (Noted this problem after I'd left the game paused for three hours.)

Saving and loading fixes this, and starts the counter again.

The changes are small however, and waiting five to ten minutes between checking won't hurt much, and can give it time to change in whichever direction it's going to move in.

In the GTA 5 Book it specifically says it changes every 30 seconds real time.