It takes 20-25 minutes to travel from NY Penn to the EWR Rail link station, from there it's seven minutes on the Airtrain until your inside Terminal C. NJ Transit Northeast Corridor and North Jersey Coast Line trains stop at EWR, not all do though so make sure the train your about to board lists Newark airport as a stop or you see the "EWR" code near the gate.

Depending on which train you take it's either one stop or two stops from NY Penn to the Newark airport rail link station, the two stops are Secaucus jct and Newark Penn.

http://www.panynj.gov/airports/ewr-to-from.html