I do think that she looks in the 3 to 4 month range. Does she still have her puppy teeth?
At 3 months Gryf was eating about 8 cups per day but if she's a mix she'll probably need less. Newfs need less food as they get older, not more. I have friends with adults only eating 2-3 cups of food per day and maintaining their pudgy selves well. Alternatively, my 7 year old Labrador only needs 1 cup of dry food per day or he gains weight like crazy and that one cup holds him at about 110 pounds, as a pup he ate at the height 6 cups per day.
Just check her waist and ribs to make sure she's getting enough. You'll know if she's not if there is very little fat felt over the ribs. Gryf is not very food motivated either and I had a lot of luck getting him to focus and eat all of his meals by wetting the kibble down early on, or putting little bits of yummy things in it, yogurt, sodium free beef or chicken broth, etc. Now he eats it dry without a problem.
Good luck with her, she's really cute!