Snippet to change Estimated delivery text based on stock quantity. ESTIMATED DELIVERY DATE PLUGIN FOR WOOCOMMERCE - https://www.pluginhive.com/product/estimated-delivery-date-plugin-woocommerce/

functions.php

/**
 * Snippet to change Estimated delivery text based on stock quantity.
 * Created at : 17 Sep 2018
 * Updated at : 17 Sep 2018
 * PluginHive Plugins : https://www.pluginhive.com/plugins/
 * Gist Link : https://gist.github.com/varun-pluginhive/1bdeb6c3b18a126cdae99ab92b451892
 */

if( ! function_exists('change_estimated_delivery_text_on_product_page') ) {
	function change_estimated_delivery_text_on_product_page( $stock_arr, $item ) {
		$product_page_text_in_settings = "Estimated delivery by [date_1] - [date_2] days";
		$text_before_range 	= "Estimated delivery by ";
		$text_after_range 	= "days";
		$quantity = 0;				// Quantity
		$text_less_than_quantity = "Estimated delivery [date_1] - [date_2] days";	// Text for less than $quantity
		$text_more_than_quantity = "delivery by [date_1] - [date_2] days";			// Text for more than $quantity

		if( is_admin() )	return;
		$stock_quantity = $item->get_stock_quantity();
		$date_range = explode( $text_before_range, $stock_arr['availability']);
		$other_text = $date_range[0];
		if( ! empty($date_range[1]) ) {
			$date_range = explode( $text_after_range, $date_range[1]);
			$date_range = trim($date_range[0]);
			if( $stock_quantity >= $quantity ) {
				$new_text = str_replace( "[date_1] - [date_2]", $date_range, $text_more_than_quantity);
			}
			else{
				$new_text = str_replace( "[date_1] - [date_2]", $date_range, $text_less_than_quantity);
			}
		}
		if( ! empty($new_text) ) {
			$stock_arr['availability'] = $other_text.$new_text;
		}
		return $stock_arr;
	}
	add_filter('woocommerce_get_availability', 'change_estimated_delivery_text_on_product_page', 12, 2);
}