Work and pumping

Can my employer dictact how many times that I can pump my breast milk? They said that pumping at 9 and at 11:30am is excessive and that all their other moms only pump once a day. They also said that between 9 and 11:30 am that I wouldnt produce enough milk anyways to pump at that time. Also I know they dont have to pay me for that time but could I use the PTO time that I have earned to put in that half hour I go over my normal breaks? I am a full time housekeeper at a hospital and work 8 and a half hours a day 8 of them paid and 30min unpaid break. I also live in Wisconsin if that helps any.