Create rule to Set Default Value
You can implement hook_commerce_cart_product_add()
The product which user is adding to cart and the line item is available with in this hook. you can fetch the Time(Interval) field value from product and save it to line item.
    
function my_module_commerce_cart_product_add($order, $product, $quantity, $line_item) {
   $commerce_line_item = commerce_line_item_load($line_item->line_item_id);
   $product_time = $product->time_interval['und'][0]['value'];
   $line_item->default_time = $product_time;
   commerce_line_item_save($line_item);
}