You could use the following code to get the reading time calculation by post ID.
`
global $readingTimeWP;
$reading_time = $readingTimeWP->rt_calculate_reading_time(1, get_option(‘rt_reading_time_options’) ); `
global $readingTimeWP; is accessing the readingTimeWP class.
Then on the next line we’re calling the rt_calculate_reading_time() function from the class and setting it’s value to $reading_time. This function only outputs the final time calculation though. It doesn’t include the labels. So you’d have to add those in yourself when you output the $reading_time variable.