Expiration configuration not working with captcha

[deepakmahajan00]

Expiration configuration not working to expire captcha. I want to expire captcha in 5 mins. For that I did like this, ->add( 'captcha', 'captcha', array( 'as_url' => true, 'reload' => true, 'height' => 60, 'width' => 200, 'length' => 8, 'background_color' => [255, 255, 255], 'required' => true, 'expiration' => 1, 'font' => 'font/captcha5.tff', 'text_color' => [42, 100, 150], 'label' => 'registration_form.labels.captcha' ) )

Can you suggest me what I am doing wrong or do I need to do something else too.