No saying exactly how long she will stay broody. But probably the better part of a month. I wouldn't let her sit on infertile eggs. I would make her get off of the nest. If you want her to let her be broody, put some fake eggs under her and let her sit away on them.
Generally RIRs are less broody. I have had some of mine go broody when I didn't have a Roo and they did the exact same thing as yours. I put them in a cage for a few days and when I let them out they joined the rest of the flock no problems. Good luck...