In a short video I just watched, it's stated that the SNC Dream Chaser can remain docked with the ISS for "up to 210 days".
What determines this limit?
210 days is a NASA's CCDev (Commercial Crew Development) requirement:
The 210 day is also know as the quiescent docking mode, during which the visiting spacecraft is not consuming its own life support provisions, but is made available to the astronauts as a functional volume and has to thus provide air circulation, light, and so on. To enable this, it requires ability to draw station's power through the docking adapter, and conserve its own autonomous power and provisions (24 hour safe haven plus time to deorbit and land). I.e. if the craft will use batteries, those have to keep sufficient charge even after 210 days on-orbit once they're disconnected from station's power, so they can power the craft for the required duration of the autonomous mode. Life support provisions, propellant tanks, and other volatiles have to maintain minimum required pressure, craft's Environmental Control and Life Support Systems (ECLSS) has to handle mission length metabolic rates, external truss will have to tolerate within design limits 210 days of on-orbit thermal cycling, and so on.
Refer to Commercial Human Systems Integration Processes (CHSIP) for more detailed information. Also of use is Human Integration Design Handbook (HIDH).