No single crew has been in the ISS, without other non-radio human contact, for 520 days. In fact, no single person has been in space for that long, period. The record at the ISS is 198 days; at MIR, 437[1].
The differences between an orbital mars voyage simulation and the ISS would be that the ISS gets resupply shipments and isn't on a 26 minute radio response delay with ground control.
http://en.wikipedia.org/wiki/List_of_spaceflight_records#Ten...