Não há data definida. Do Debian Wiki :
oldstable is a codename for the previous Debian stable repository, as long as security updates are provided. It is not just a name of the old stable version(s) of Debian.
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Q) How long will security updates be provided?
The security team tries to support a stable distribution for about one year after the next stable distribution has been released, except when another stable distribution is released within this year. It is not possible to support three distributions; supporting two simultaneously is already difficult enough.
Isso significa que Wheezy terá suporte por cerca de um ano após o lançamento de Jessie. No momento da escrita, não houve data definida para o lançamento de Jessie.