How long are Debian stable releases supported? like Debian Wheezy

There is no set date. From the 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.

This means that Wheezy will be supported for about one year after Jessie's release. At the time of writing, there has been no set date for Jessie's release.

As discussed the Debian releases are supported for about 3 years, then one year after the next stable release as shown here: https://wiki.debian.org/DebianReleases

There is now a LTS repository of Debian which you can swap over to get 5 years out of a release, ref: https://wiki.debian.org/LTS/

The LTS repo takes Debian 7 "Wheezy" support to May, 2018 & "Jessie" to May, 2020.