Stafs are super active and will eat a lot because they naturally have high metabolisms. If you haven't checked her for worms, you may want to do that, this would make her abnormally hungry. Also, feeding once a day is not enough for most dogs.
My dogs get breakfast of two large biscuits (or a piece of ww bread or toast with a dab of peanut butter) then their big meal at dinner. At dinner my larger dog (about sixty pounds) gets a cup and a half of kibble with 1/2-3/4 cup whatever "treat thing" they get that night, the smaller dog gets a cup of kibble with a little less "treat thing". The "treat thing" is either canned dog food, a rice/meat mixture I cook for them or a smaller amount of canned cat food. "Treat thing" is only to get the larger one to eat, she refuses to eat dry kibble, period. Her problem is that she won't eat enough to maintain a healthy weight if I don't make it very tempting.
If' I'm home during the day they also get dog treats or tiny bits of whatever I eat as treats.
I know, I know, my dogs are spoiled rotten.
Different dogs have different temperments regarding eating. My smaller dog (a healer/chow cross) is a total chow hound and will eat anything I put down, including dry kibble. She will blow up like a balloon if I don't monitor her weight and her behavior is why I even measure the food. My larger dog (husky/wolf cross) is a very picky eater, really just wants human food and will turn her nose up at canned cat food if she thinks I have something better on my plate.
The real measure of how much a dog should be eating is it's weight and health. When rubbing their side you should be able to feel their ribs under a layer of skin and flesh. You should not be able to see their ribs, although on a staffy with that short tight coat you will likely be able to see some ribs slightly defined unless they put on a fair amount of weight.
I have had other dogs, and some dogs just can't be left to free-feed or they get really fat. Other dogs never seem to overeat.
My suggestion is to use a cup measure as a scoop so you know what you are feeding, change things as you feel you need to and watch for the results. If she gets fat, cut back the food, if she's skinny, give her more.
It's also very important to be feeding a high quality food. Different brands are available regionally, you'll have a better selection and better quality available at a pet or feed store than the grocery store. I prefer the rice based foods to the corn based foods for too many reasons to go into here.
Best wishes and good luck!