While I don't know the science involved, my trainer prefers that I get 35-60 minutes of cardio in each day. When I was actively losing (as opposed to kind of trying to lose as I am doing now), I went for 60 minutes, although I changed up activities at 30 min. because, frankly, 60 minutes on the elliptical, bike, or rowing machine was interiminable. I could do 60 min. on the treadmill only if I'm interval training.
But what is key, IMHO, is what will you stick with, and what works well with your schedule. When you're starting out, 60 min. of cardio (I'm assuming you're talking cardio??) at one time is an awful lot.
Women who behave rarely make history.