calc-read-time.php

<?php

/**
 * handle the calculation
 * @param  integer $seconds [description]
 * @return [type]           [description]
 */
function rkv_calc_read_time( $seconds = 0 ) {

	// calc the minutes
	$minutes	= floor( $seconds / 60 );

	// if the time is less than a minute, return that
	if ( $minutes < 1 ) {
		return __( 'less than 1 minute', 'rkv-reading-time' );
	} else {
		return sprintf( _n( '%d minute', '%d minutes', $minutes, 'rkv-reading-time'), $minutes );
	}

}

display-read-time.php

<?php

/**
 * display the estimated time to read the content
 * @param  [type] $content [description]
 * @return [type]          [description]
 */
function rkv_display_read_time( $content ) {

	// fetch the global post object
	global $post;

	// check for our meta key
	$seconds	= get_post_meta( $post->ID, '_seconds_read_time', true );

	// bail without the key
	if ( empty( $seconds ) ) {
		return $content;
	}

	// get our calculation
	$readtime	= rkv_calc_read_time( $seconds );

	// create a prefix
	$readprfx	= __( 'Estimated read time:', 'rkv-reading-time' );

	// make a fancy box
	$readbox	= '<p class="estimated-read-time"><strong>' . esc_attr( $readprfx ) . '</strong> ' . esc_attr( $readtime ) . '</p>';

	// send it back before the content
	return $readbox.$content;

}

add_filter( 'the_content', 'rkv_display_read_time', 10 );

save-read-time.php

<?php

/**
 * store the estimated time to read the content
 * @param  integer $post_id [description]
 * @return [type]           [description]
 */
function rkv_store_read_time( $post_id = 0 ) {

	// run various checks to make sure we aren't doing anything weird
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
		return $post_id;
	}

	// only save on posts. would probably set this to a filter
	if ( get_post_type( $post_id ) != 'post' ) {
		return $post_id;
	}

	// fetch the post content and get the word count
	$content	= get_post_field( 'post_content', $post_id, 'raw' );
	$wordnum	= str_word_count( strip_tags( $content ) );

	// set the average reading time at 120 wpm
	$avgtime	= apply_filters( 'rkv_estimated_reading_time', 120 );

	// calc the total seconds to read
	$seconds	= floor( (int) $wordnum / (int) $avgtime ) * 60;

	// update the post meta
	update_post_meta( $post_id, '_seconds_read_time', $seconds );

}

add_action( 'save_post', 'rkv_store_read_time' );