In my experience, it usually takes a couple of hours. However, sometimes the A-Records for a sub-domain get 'stuck'. In such cases you need to contact support, outline the situation and they will run a manual DNS update on your domain, which should resolve the situation.
Edit: you can verify if this is indeed your problem, by going to the Manage Domains page of the panel, clicking on the small [DNS] link next to the parent domain and looking for the A-Records associated with the sub-domain. If you have removed the sub-domain, they should not be present.
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