I am Ubuntu user; while I know that Ubuntu 12.04 is LTS and will be supported until April 2017, I'd like to know what the projected support lifetime of Debian Wheezy will be.
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.
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.
Very hard to say. I tried to get a general lifetime for Debian myself and ended up with a rough estimate of currently three years support.
This is much less than the five years for Ubuntu LTS. My feeling is that Debian will go into that direction, too.
If they want to stick to their policy of "not more than two active versions at the same time" (see Joseph's answer), they should release a new major version only every four years.