When is analogCallback called?

What controls when analogCallback is called after you run: 

analogInReader.BeginReadWaveform(-1, analogCallback, myTask);?

I found a webpage on this site that discusses the buffer size that Nidaqmx allocates depending on you samples rate:

http://zone.ni.com/reference/en-XX/help/370466V-01/mxcncpts/buffersize/ .

But what about the last argument when you set up the timing of the channel?  As in:

// Configure the timing parameters

myTask.Timing.ConfigureSampleClock("", ADParams.sample_rate,SampleClockActiveEdge.Rising, SampleQuantityMode.ContinuousSamples, 100000);

Here I set the buffer to 100,000 samples? Will this be used or does Nidaq pick its own value?

After how many samples will the callback be called?

Thanks,

Dave