There is no official word if his time in the group has an expiration date, most likely he will stay in Nexus until they disband or kick him out.
UPDATE: Cena will be in Nexus up until the Survivor Series 2010 PPV, November 21st 2010. If Wade Barrett wins the WWE Championship from Randy Orton, he will be free of Nexus. But, if Orton retains & Barrett doesn't get the championship Cena is "fired" (storyline-wise) from the WWE.
Either late 2010 or early 2011 to keep the storyline quite long.
John Cena has been wrestling 10 years
eight years!
cena's house ifo
9 years
from 10 to 2 (am)
23 inches hard
John Cena is "fired" although not for long. It was only a storyline so he is not actually fired and he is currently an active wrestler.
i think a week there no wwe without john cean
around 3 years
380 days
1967