What is Voice Generation Time (VGT)? How is the VGT calculated? 

VGT is the time that is taken into account based on the duration of the audio file for which the speech is being generated.

Voice Generation Time (VGT) is calculated every time you generate voice from text using any of the three options to generate audio (Project, block and sentence level).
The generated audio file's duration in seconds is deducted from your available balance which can be tracked from the top of your screen in the Studio.
This is irrespective of whether you download the audio or not.
To keep your voice testing usage low, we recommend that you test your voice with smaller paragraphs.