Safest bet would be to stretch it out over 7-10 days. Divide the ammount that you feed her into 7 parts and replace one seventh each day with the new food. If she has a bout of the runs one day, back up to the mixture of the day before and remain there until her stools are normal.
Of course if for whatever reason she should react adversely (trust me you'll know!!) then discontinue it completely and return to the old food, but Nutro's is a great food, that's highly unlikely.