The two Americans onboard the ISS, Bowersox & Pettit, plus their Russian colleague, have indicated
that they are willing to stay onboard
the ISS for upto and over a year,
considering that the ISS schedule
has been turned on its ear due to the
Columbia tragedy.
I personally don't think that NASA
will allow them to stay in space past this
November, when the next Soyuz lifeboat
has to be replaced.
That would leave Bowersox and Pettit
11 months in orbit.
11 months is 11 months.
That's roughly the round-trip time
for a manned mission to Mars by VASIMR propulsion system.

God bless them for their zeal, their
passion for space and courage.

Jacob