Currently, Debian testing is in a freeze state. This means that new uploads must be approved by the release team, and generally must fix RC (release critical) bugs. It is very rare for the release team to accept new upstream releases (rather than patches specifically for RC bugs) after the freeze.
So the answer to this question is after the following has occurred:
- The Mono team packages and uploads Mono 3.0 to unstable
- Wheezy is released as stable and Jesse becomes the new testing
- 2-10 days have passed since the upload to unstable (depending on urgency set on the package).
In addition to this, if a RC bug is filed against the unstable package before it migrates to testing, the RC bug will bock migration. The severity of the bug will need to be downgraded, or a new version of the package which fixes the RC bug will need to be uploaded.
Outside of a time in which testing is frozen, the answer to your question is "2-10 days after the maintainer or team has time to do the work and upload to unstable". Maintainers or teams own packages in Debian, and they are all volunteers, so it is really dependent on the individuals involved.
Unfortunately, I do not know of any direct sources where this process is clearly laid out. I have this knowledge from years of working with OS and lurking around the development community.