The blog content is a string. You can use PHP function to count words in a string
Next, is to do a math floor calculation.
Words in a 1 minute speech 130 words.
Words in a 2 minute speech 260 words.
Words in a 3 minute speech 390words.
Words in a 4 minute speech 520words.
Words in a 5 minute speech 650words.
You may compose the formula yourself.