iPhone Remote Audio Unit callback

Question

I am looking at Audio Unit Development Fundamentals. Assuming

   UInt32 sessionCategory = kAudioSessionCategory_PlayAndRecord;
   status = AudioSessionSetProperty(
            kAudioSessionProperty_AudioCategory,
            sizeof(sessionCategory),
            &sessionCategory);

with sample rate 44,1000 and allocate the buffer large enough to hold 1 second recording

1. How often the recording callback method will be called?
2. Once per second?
3. What's the algorithm to decide to call the callback method?

Thanks in advance for your help