In cocos2d, when you create a timer with the default interval parameter (space):
[self schedule:(update)]; //Not a 100% sure this code is correct, but you know what i mean
What interval does the timer work, I think its 1/60, but I'm not sure.
It depends on the settings of your CCDirector. If you use
[[CCDirector sharedDirector] setAnimationInterval:1.0/60];
or
CC_DIRECTOR_INIT();
then the update method is called every 1/60 of a second.