Is there a known date/timeframe when python 2.7 will not be supported any more in favor of python 3?
PEP 373 (Python 2.7 Release Schedule) is the official source for the kind of information you asked for.
It currently says "Planned future release dates:"
Also, it says "The End Of Life date (EOL, sunset date) for Python 2.7 has been moved five years into the future, to 2020."
Edited in April 2014, according to http://hg.python.org/peps/rev/76d43e52d978