There should be instructions on the bag about how much to feed according to weight. You can use that as a guide, and then adjust to be sure your dog maintains normal growth and has normal poos.
(And look out, you might get a little flamed by people who really believe it's better for your dog to get a high quality food. OK, I am one of those people! But I also think you have to pick what's best for your own dog, for instance my dog doesn't tolerate grain free, it's too rich. OK, sorry again, I'm hijacking your post
And don't let the flames dissuade you from asking questions! There is excellent info here.)