JS - community manager at Insomniac games provided some info on the longevity of co-op mode in Resistance .
According to him each map has different areas with 6 maps being available for the co-op mode of Resistance 2
Each co-op campaign with an individual setup takes between 15 - 20 hours to complete
"There are six locations, each has quite a few different areas, and a single-playthrough of any will not show you nearly the entire map or all of the different set-ups on it due to the randomization of the playpath. It probably takes between 15-20 hours to see every single co-op set-up and encounter. "