the problem is nutrition, muscle mass, and bone density. think about sitting infront of your tv 24 hours a day for 21. you get up once a day to use the bathroom, eat (what the rest of the family has left as leftovers), drink something and streach your legs. you get about 30 min to do all this then you sit back down.
a chicken is built to do this for about 25 days then spend 6 weeks with chicks and 1 to 2 weeks laying before setting another clutch. many will set till they can't set any longer then break the nest and try again later after they have their health back. them setting for 42+ days is a streach. she may be able to set that long, but to me that is asking a bit much. she may try and then decide she can't and you are stuck with eggs that are half way through and a weak hen. when this has happened to me, i have made the call and forced her to break the nest. sometimes just sticking them in a box with no nesting material will do it, but if you add a roo he will breed her every time she gets still. it doesn't take long for her to decide that setting isn't for her for a while and give up for a few weeks.