Determine the time length of audio training samples?
It's a very bad idea to stack together consecutive frames in a single vector. By doing so, you are training a classifier to recognize the exact same sound as the one in your training database, pronounced at the exact same speed.

For your application (which appears to be speaker identification), you should consider each individual frame as a training instance (aka "bag-of-frames" models). That is to say, each training sample will give many 12-dimension feature vectors.